Constant restarts

what brand PSU is it?

and how old are the parts?

I had a similar problem were my computer would just re-boot randomly without an error in my events or a BSOD, and it ended up being a USB problem so I started unhooking my USB connections one by one until the problem stopped. It ended up being the USB connection on the motherboard for my card reader, so I shutdown unhooked the card reader rebooted, shut back down, reconnected the card reader and rebooted and everything has been fine ever since. But it could still be the PSU or HDD, but the USBs are a good place to start.

If it makes it to a certain point and then stops, I would check the RAM first. It may be hitting a bad sector at that point and crashing. So try flipping the modules, or better yet, booting with one at a time.

One other thing you might check, with a system that old, is that the motherboard's CMOS battery is dead. That basically means the BIOS will reset to defaults every time you turn on the computer, so if there's something like your RAM not being standard voltage, or the video card conflicting with onboard graphics, that could be messing things up. Replacing the battery for like $3 is worth a shot.

If it's neither of these (I believe in looking at the cheapest problem first), then yes, either a PSU problem or a HDD problem could be responsible. I'm not inclined to blame the PSU if it's crashing at a certain point every time, though, unless that point was in the middle of a game. On boot, it's most likely a RAM or HDD issue ... if it's a hardware issue, that is. It's also quite possible with an older system that you just have too much Windows rot and that's making it crash.

Sounds like the PSU is out.