I have an old HP at home, maybe 5-6 years old. The PC tower hasn't been responding lately, like turning on, though the monitor does. What could the problem be if I didn't notice anything wrong with it the last time I was home? Power supply? That's the only amateur guess I have.

Simplest guess is usually the best guess. When you say that the monitor turns on, though, do you mean that it responds when you try to power on the computer or just that it turns on when you turn on the monitor?

If the monitor doesn't respond when you try and power on your computer, your best bet is going to be replacing the power supply.

I mean that if the cords for both the PC and the monitor are plugged into the same surge protector and the monitor turns on and off but the PC doesn't respond to anything. The PC never turns on so the monitor doesn't turn on fully; when I say the monitor turns on I mean I can press the button to turn it off or turn it on and it will go into stand-by mode since the PC isn't on.
Good thing it's Black Friday tomorrow because CompUSA has a 350-watt PSU for $5AR.

That's what I figured you meant. New PSU it is.

Also check to make sure there's no additional power switch in the back of the tower that got flipped. Sometimes there is a manual on-off switch back there, buts its rarely actually used.

The only other switch I saw was the voltage switch. I've got that $5 PSU shipping here now so hopefully it'll fit and all that.

Well that PSU I got doesn't work with my old PC. The old PC's motherboard connections or whatever have less slots than what's on the PSU's cords. Also, the PSU says it's got SATA connections and I wasn't sure but it didn't seem like they fit with the hard drive and CD drive and all that in the old PC, which are IDE cable'd.
Is there anywhere I can get like...an old PSU to work with this PC?