Windows acting odd

Ok, so I just reformatted my computer, and installed a fresh version of Windows XP Pro. However, I'm having this one problem. Sometimes, programs will appear to load, but never actually run. Windows'll display the hourglass pointer, but it just goes away after a while. This happens with everything: Games, word processors, Internet browsers, etc. I've checked every system log I can think of, but there doesn't seem to be any indication of an error. I was getting this problem before I reformatted, in fact, it was the very reason I reformatted, so it might be a hardware/driver error.

My PC's specifications are as follows:
Mobo: Asus AN78X-X
Processor: AMD Athlon 2400+ 2GHz
Video card: ATI Radeon 9600XT-128
RAM: I don't know the manufacturer, but I have half a gig of it.
Hard Drives: Western Digital Caviar WD120 120GB/Seagate Barracuda 200GB
Sound Card: Creative Sound Blaster LIVE!
OS: Windows XP Professional SP1

>>6
The problem is, Task Manager won't even open when the error occurs.
>>7
Using Firefox. NO I WILL NOT SWITCH TO OPERA UNLESS IT SOLVES THIS PROBLEM. DON'T EVEN TRY.