computer dies when I restart

I have a windows xp pc.
Usually I leave my computer on, downloading things off BT and the like.  For months there were no problems.  Tonight, twice in a matter of hours, I restarted my computer and when it boots up again it dies abruptly.  Mind you, the computer is working perfectly fine before the restart.
The first time I just hit the power again, thinking I hit "Shut Down" instead of restart.  It dies again before any text can load on screen. 

I open it up, take a look at the parts (I have intermediate experience at best) everything seems to be in order.  I feel the power box, the thing that the power goes into, and its hot.  I'm guessing its hotter than usual and I think this is my problem and its happening because I live in the desert (Las Vegas) and I think the heat is getting to it.  Although this didn't happen last year, this is my only guess.

I leave it off for over an hour, and have a large fan blow into it to cool it off the entire time.

When I turn it back on again its fine, completly normal.  At one point I'm using a program and it dies on me twice, I'm guessing I was multi-tasking too much and using too much memory.  I restart the computer and have the same problem again.  Its off right now and I haven't touched it since (this is a different pc I'm using to write this message).  I don't know what the problem is that's preventing it from booting up.  I worry its a bad power supply thing, that bugs me b/c I just installed a new one last year, also replaced the processor.

What can I do to fix it?  Thank you

Installing Linux and/or Opera will fix your problems.  It will also fix many problems you didn't know you had!

I hope my advice was helpful.

Be sure to FORMAT too. And stop using Internet Explorer.

Okay, ignore the trollfags, you have a hardware problem. It sounds like your PSU is dying, though it's also worth checking the capacitors on your motherboard for the dreaded capacitor rot (I've seen many a machine display similar symptoms, and when you open them up the capacitors are invariably drooling brown gunk out of the orifices).

hmm.. I feared as much about the Power supply thing dying.  Now about the capacitor rot issue, they'd be visibly leaking as soon as I open up the PC's case or would I have to do something further like wiggle the capicotors themselves to see if they leak?  Right now there's no brown liquid and I don't want to touch anything unless I know what I'm doing.  Is there a way to test if the PSU is dying?

>>2
Opera sucks sure there's an ebuild for opera but it just get dropped to /opt, it's statically linked, and it's CLOSED SOURCE, which means that it is a BINARY package.

A couple months ago my BitTorrent downloading computer went a bit nuts. It kept rebooting and bluescreening. Turns out the massive BitTorrenting (having 9+ open at once) created 94% file fragmentation. This was on a seperate drive from the OS. I moved all the data from the fragmented drive to another drive, formatted the first drive and moved everything back. Haven't bluescreened or been in a boot loop since. I defrag it every now and then too.

>>5
Borrow a PSU from your friend and see if it works with that.
As for capacitors, that problem would be obvious from the first look.

thanks, I got a new PSU now and the comp makes much less noise, like it normally should and did long time ago, and its much faster.  Thanks all

>>9
if you got a psu with the same wattage as the previous one, your problems might recur after a time.  you might need a more powerful psu for what you are running.

Ah thanks for the warning, I did consider this tho. 
old one 300W, new one 500W.  I should be good for a while :)