Windows won't boot??? blue screen

[#1]

Hi All.
I have a bit of a problem and would be most grateful for any advice/help.
My brother has decided to go on holiday (lucky him), however before he went he dropped off his pc and asked if I could have a look at it as it doesn't work (nice).
I switched it on to find that when it reaches the Welcome screen the computer screen goes black, then reboots, never going into windows.
I have manged to get it to boot in safe mode, and then unclicked the automatic restart option in the Start up and recovery section of System properties (from control panel), I then rebooted the PC.
The same happened got to the Welcome screen then screen goes black and tries to reboot, however this time I get a blue screen which states "STOP: c000021a (Fatal System Error)"
I understand that this error happens when either Winlogon.exe or Csrss.exe processes fail, due a third party drivers or apps.
I have tried selecting Last known Good Configuration option, but this hasn't made any difference at all.
The question is How do I sort this problem. As i don't know what drivers/apps have been installed/removed, I don't really know where to start.
Like I said at the start of this post, I would be most grateful for any help/advice/guidance with regards this problem, I have searched on the internet but not really found a solution.
Thanks in advance.

[#2]

I assume you are using XP. If you have a suitable XP install disk you could try a repair install as described here http://www.informationweek.com/news/windows/showArticle.jhtml?articleID=189400897

[#3]

You could also try using system restore to roll back to a previous system state, this will also tell you what changes have been made such as whether service pack 3 was installed recently.

[#4]

Pressing F8 at startup and selecting enable  boot log is a good option as this will create a log of what loads and you will see were it fails. Once its run you will have to boot into safe mode to read the text file to see were the problem is the file Ntbtlog.txt is in the windows directory.
A good article by MS on boot problems is herehttp://technet.microsoft.com/en-us/library/bb457123.aspx
Its also worth running chkdsk c: /f to see if there are any disk errors it may say it cant run this and ask if you want to run i ton next boot if so say yes