Blank Desktop

[#1]

Can someone Help !!
I have a 4year old Dell Dimension 5150 Desktop with a CRT monitor, with loads of disc space left.  I have Windows XP with service pack 3.  I have a paid for version of Spyware Doctor which is up to date and Registry Mechanic. I do frequent disc clean ups and defrags ;  I have had no trouble up until recently. Then several times after booting up successfully,  I have entered my Administrator password and a totally blank desktop has appeared with no icons or notification area.  I have then been unable to bring up the Start menu to restart it and have had no choice but to shut it down with the on/off button.  I will then restart it and everything seems normal.  Also several times when I have left an application open whist I have gone off for 10 minutes I have returned to find it has  locked up and the notification area has gone missing again. 
Grateful for anyones opinion about how I might tackle this problem as I know it is not a good idea to keep shutting it down with the on/off button. 
Nick 

[#2]

That sounds to me like explorer hanging.  As well as being the main file browser, it is also the primary application (the shell) that provides all the desktop functionality,   if it breaks,  the desktop goes with it.
If your lucky,  the three fingered salute (Ctrl+Alt+Delete) will either take you straight to the task manager,  or to a dialog with it as one of the buttons.
In there, select the processes tab,   find explorer.exe, select it and kill it.
The select  'File->Run' and type explorer.exe then hit OK, if things go according to plan, the desktop should appear.
There should be a shutdown option in the task manager too,  which is preferable to the power switch.
As for what is causing it,  that's a different ball game.  You could try looking at the event logs to see if there is anything useful in there  (right click on 'My Computer' then select manage, go to Event Viewer)

[#3]

I had that problem on my mothers laptop a couple of weeks ago, and what kmilburn posted solved my problem of accessing the desktop. As soon as I got the desktop back, I backed up all the files onto an external drive. After much searching on Google, I found it was a corrupted registry which happened because the laptop was infected with a worm, followed the advice and made it worse. My solution was to reinstall XP

[#4]

It's worth trying booting in to safe mode. Do this by tapping the F8 key several times during the start of the boot sequence then select the option for safe mode. If you find that you have a normal desktop in safe mode rebooting afterwards might fix it for a normal booting.

[#5]

Thanks for all the help.  It has behaved itself for a couple of days now.  I've made a note of all the suggestions so far and will apply them next time that I have this problem.  I have started backing up all the important files that might get lost if it crashes.  As I am not particularly computer literate, as I expect is quite apparent, delving too deeply into the operating system and programmes is a bit beyond me, I am always terrified that I will delete or alter something I shouldn't  !! 
Nick

[#6]

Once you backup your files and if you have an installation disc, muck around as it is a good learning curve (at times) and you can always start from scratch (time consuming though). If it happens again you can always try "System Restore"  you can find it from Start > Help & Support > System Restore and just take your computer back to an earlier time when it did work (doesn't always remove the problem though)