Strange keyboard and mouse problem

Hey guys,
since i installed windows 7 on my pc (formerly win xp) i've got a really strange problem:
At first everything works fine, but after a few minutes or a few hours keyboard and mouse turn off. Not necessarily at the same time, but in the end none of them is working. But apart from that everything works fine, the computer doesn't freeze. Keyboard and mouse are both old PS/2 devices.
Has anyone got an idea how to solve this issue?

It might be something with the PS/2 port, or perhaps something with the keyboard/mouse.

Have you tried another keyboard/mouse? Are their drivers up to date?

Well everything worked fine with Windows XP, so i don't think it's a hardware problem. And i've tested another mouse and keyboard already, that didn't change anything.
The drivers are just the standard mouse and keyboard drivers that came with Windows 7.

mpx;
Hello and welcome to the forums.
Double check the properties of both devices in device manager and see of either or both contain a power management tab. If so select it and uncheck the box that says something like "Allow computer to turn of device to save power"

The box is greyed out and unchecked for both devices. But thanks for your help anyway!