Weird standby

I've got a problem with my WinXP SP3 computer. Sometimes when I try to put it on standby, the computer goes to the "Windows is preparing to standby..." screen and freezes there. My box will be "preparing to standby" for hours, or even days, if I would let it. This will happen both if I tell it to standby and if Windows goes into standby by itself after X minutes.

Also, sometimes when I start the computer up, I can begin to use it, and then it randomly reboots, like if there were a power blink, but there will have been none.

Help, please?

 

Does this happen if you try to hibernate the computer?

 

No, hibernating works really well. It's quick and gives me no problems.