Why is my computer running soo slow ???

[#1]

Now I don’t normally ask for computer help, but this one’s got me stumped…  Someone out there may have the answer in which case I’ll owe you a pint…
The computer details are as follows:-
Dell Dimension 9150 running XP MCE
Pentium(R) D 3.00 Ghz processor
4 Gb of ram
Hard drive is a sata 250 Gb., ~30 Gb used.
The computer has functioned well since I purchased it some years ago but recently it appears to have slowed down considerably – to the point of almost being unusable for 3 – 5 minutes after boot up.  It actually boots to the desktop in under a minute but getting it to do anything for the next few minutes is problematical.  My anti-virus (free Avira) updates on boot up but according to Windows Task Manager only uses ~30% cpu power or less so that should not cripple the computer surely?  In any event I’ve uninstalled Avira and it’s made precious little difference. 
Defragged and checked the hard drive for errors using Microsoft’s tools. I run Comodo Firewall and MalwareBytes anti-malware, these are the only two things that run on start up.  I also occasionally do an on-line scan using Trend Micro or something similar.  I sometimes run HijackThis which I’m not too familiar with but it doesn’t appear to flag up anything obvious.  CCleaner, Revo Uninstaller and Dr Web are installed and I run them every few days. I’ve replaced the nVidia graphics card with a known good one as this is a well documented problem with these machines.  The cooling fan was running very fast and a major clean up inside the PC, including new heat sink compound between the cpu and the cooler has cured that.  No improvement.
This pc used to run well and could open and run multiple programmes/screens but now it’s as if it has run out of breath – its specs are still reasonable and I don’t want to pension it off if I don’t have to… The intriguing thing is that when it’s running slow Windows Task Manager never shows the cpu usage as being high but it’s as if something is slowing it down for the first few minutes but not registering in Task Manager…
Yes I know – junk XP and install W7, and I may have to do that in the foreseeable future but for now I’d just like to get my old XP back…
Apologies for the long diatride but this doing my head in

[#2]

Take a look in CCleaner ======> Tools =======> Startup =======>Windows & Scheduled Tasks
It might show anything that can be Disabled / Deleted that is possibly causing the issue(s)
There is another possibility of Windows Updates causing issue(s) as documented via these forums
If in any doubt what to Disable / Delete
Post them here & we can advise further

[#3]

Thanks for that.
In Windows it shows Comodo Firewall whilst Scheduled Tasks shows TuneUp Utilities.
I've used both these softwares for years without any problems (that's obviously not to say that one of them has not gone bad...)  TuneUp is a collection of maintainance tools and automatically does a quick clean.  I can easily disable it - or even uninstall it.
Since my initial posting I've disabled jqs.exe (which is related to Java) which was shown running in Task Manager and I thought for a minute things had improved.  When I re-booted the PC it got to the desktop quite quickly - ~30 seconds but Firefox took ~60 seconds to load. 
I wonder if this problem is caused by more than one thing?
Appreciate your time.

[#4]

It could very well be caused by more that 1 thing tbh
You could try a clean boot
See http://support.microsoft.com/kb/310353
&
Advanced Clean Boot http://support.microsoft.com/kb/316434
To help see what is going on

[#5]

TORPC - Had another root about this morning - before following your advice and trying a clean boot.
I uninstalled Avira a few days ago but I notice there are three references to it in Services, I can disable 1 but I'm forbidden access to the other 2...  I downloaded an app from Avira this morning to clear any remaining Registry references to their software and this cleared a ton of stuff but there were some that it couldn't clear.  The Services references to Avira remain and I can do nothing with them, though they may not necessarily be the problem.
I manually started XP using msconfig.  On exiting I get  'Access denied error was returned when attempting to change a service...'  or such words.  The PC starts and I get the impression that the Task Manager shows less entries under processess.  Firefox loads quicker in this state.  I think I'm right in saying that System Restore is disabled during Selective Startup so i get a bit jittery playing around in there!
Seriously it does  'look'  as if disabling the Windows services speeds up the browser.  Is it now a case of disabling them one by one?  There are dozens.
I fine tuned Services ages ago when I installed XP, disabling all the unwanted ones.  This has not been messed with since.
Thanks again for your help

[#6]

These things can creep back in, especially after updates etc

[#7]

I ran Avira's uninstall app in safe mode and managed to clear the remaining references to their AntiVirus.  It was just a side issue really as I didn't expect it to be affecting Firefox's slow start and indeed the problem still remains.
I disabled Comodo Firewall and Firefox appeared to be opening quicker, re-enabled it and I'm back to square one - slow start up.  SO I uninstalled Comodo and did a clean re-install.  Firefox still behaving well when suddenly it's back to it's old tricks.
This problem appears to be intermittent and not easy to tie down to anything...

[#8]

Might be totally unrelated but I have got so sick of Firefox opening slowly that I have changed to Waterfox which is much snappier

[#9]

@Jim ,, yes waterfox is good ....... but i cant launch bf4 on a 64bit browser 

[#10]

Thanks guys for the various suggestions...
Nothing seemed to have much effect - well removing programmes did and it was cumulative - the more I removed the faster it got, but unfortunately I needed some of them!  I suspect it was the hard drive getting a bit tired.  This Dell has 2 SATA hard drives (C & D) so I eventually installed a clean copy of XP on the D drive and swopped their connections on the motherboard so that now my D drive is my C - bit confusing really but essentially i have a clean version of XP on a new drive and I get the impression things are a bit better, certainly the old drive had got a bit noisy these last few months and this new one is nice and quiet.
I decided to ditch Avira Anti Virus for Avast and I suspect that might have speeded things up as well.
Maybe the time has come to drag myself from the last century and treat myself to a new machine with W7 and better specs...
Thanks again for your input.