Computer crashing

Every once in a while my computer crashes, but I'm not sure what it could be. I'm thinking it's either the HD or the power supply. When it crashes I have to flick the power switch in the background and keep it off for like 10 seconds before it'll work again.

Also, when I'm starting my computer up in the morning it usually doesn't get all the way through it, not even to the windows screen. First I can hear it on, but the monitor won't be working. So I turn it off, turn it back on, it crashes. Go through this a few time before it sits at the screen where it checks my processor/ram. I let it sit there for a few minutes and then shut it off and turn it back on, then it'll *maybe* work for a while. If it's really working, I'll be able to play games. Half the time it'll start up to desktop and I can use pretty much everything but I won't be able to play games. I'll open the game, and go to load a map up or whatever and then crash.

So, what do you suggest I do? I might be able to borrow a friends HD and PS to check what's wrong, but I'm not sure. What do you think the problem is? My PS, HD, or something else?