Unexplained, intermittent crashes

Ever since I bought my PC in May 2008, I was encountering some very strange crashes. After every first startup of the day, in about 10-30 minutes, the computer completely crashes. Screen freezes, no mouse movement, and also the last notes of sound are repeated in the speakers like an endless noise. After hitting reset the PC reboots and it is working perfectly as long as I want even when playing 3D games or watching movies. Every day when I sit down to my computer after work and start it up again, the same thing happens. I noticed that playing games makes the crash come sooner than just browsing the internet or something, so I got used to playing a DeathMatch in Unreal Tournament 2007 just to let it have its freeze and then start doing whatever I want without having to worry about the crash.

Some more remarks about my problem:
- the crash occurred even when not playing or watching movies, like during browsing, but as I said, video-intensive activities made it come sooner
- I am currently using Windows XP Pro, but this happened also under Vista
- after the freeze and resetting, I noticed some strange behavior during the startup: sometimes the whole computer pauses for 2-3 seconds after going off and before actually turning on again; other times the windows loading screen (when the progress bar is moving) is very odd: it pauses and its colors are dimmer, after 10-20 seconds it boots up normally
- recently (and only recently) sometimes the crash happened either after several hours of using the computer (without gaming) or did not happen at all - this is the worst part of it all, since now I cannot "make" it crash and always have to worry about it

What I did so far was running a memory test which did not produce any errors, and I also tried to remove 1GB of the memory, but the same thing happened. I also looked at the event logs but could not find any clues, I might have missed something though.

Does anyone have any ideas why this strange crash could occur?

Any help would be much appreciated, and sorry for my English

Number one suspect is shorting within the power supply. In your position, I would immediately replace the power supply with a more capable and better quality one. It is not an expensive nor difficult thing to do. Just make sure the new one is of the right specification and size and gives all the neccessary type of power connections.

Next suspects would be overheating video card memory, overheating CPU, motherboard fault. If you have an IT friend at work or know anyone who could loan a video card, you could also test that by replacement. CPU and motherboard are more tricky, but you can install temperature monitoring software and watch the motherboard and CPU temps with one eye whilst trying the stress the PC.

Thank you, gbhall .

Interesting what you say about the power supply because I specifically paid more to have this "CORSAIR VX450W" PSU which was supposed to be better quality. Can it be that the 450W is not enough? (By the way, should not that produce recurring crashes, not only one during each session?)

Anyway, I will try out your suggestions to get to the bottom of this.

Thanks again!