PC keeps randomly restarting

I have yet another problem with my computer. It is a desktop pc- windows xp, 3 ghz Pentium 4, 1.5 gb ddr ram, 160 gb hard drive, geforce 5200fx, 300 watt power supply.

My PC keeps on randomly restarting. There is no specific thing that triggers it. I’ll give you a timeline of events.

Around 5-6 months ago, my computer would sometimes restart in the middle of the night while I was sleeping, and when I came home later the next day in the evening, I’d find that my computer had restarted. This was pretty rare, however; it happened maybe once a week. Then, starting around 1-2 months ago, my computer would restart more frequently. I would be watching a video, and about 30 seconds to 1 minute after I opened the video, the computer would restart. Sometimes it would restart when I was just browsing the internet, or when I was doing nothing and no programs were open. One time, I put in a portable hard drive and started to transfer files to it, and the computer restarted 1 minute later. The portable hard drive connects with 2 usb ports. It never restarts when I put a flash drive in there and transfer files. Starting about 1 week ago, it hasn’t restarted. I have played video files, and it never restarted. I also recall that whenever there was a power outage and my computer shut off because of the power outage, when I turned it back on, it would restart while the black windows loading screen was showing. Then when it booted up for a second time, it didn’t restart. The same thing happened when I unplugged my pc and plugged everything back in and booted it up (I moved, so I had to set up my pc again).

Another thing worth mentioning- I thought it might have been my power supply, so I replaced my 300 watt power supply with a 250 watt power supply that I took out and haven’t used for about 3-4 years. When I booted up the computer and logged into windows, about 30 seconds to 1 minute later, the computer would restart.

What seems to be the problem and what can I do to fix it?

Thanks.

Jam it back in, in the dark.

Doesn't the computer sometimes restart to prevent damage to the components? If I set it so that it won't restart during an error, could it potentially cause any problems?

There's nowhere I can't reach.

It is an HP. It originally came with a 250 watt power supply. Unfortunately, it has very unique dimensions for the power supply, so the highest power supply I found was 300 watts.

This thing is sticky, and I don't like it. I don't appreciate it.