PS2 keyboard sporadically stops working

Hello,

I've AMD Athlon 3600+ on Asus A8N-E Motherboard with 1GB RAM.
Recently I've encountered a weird problem: from some unknown reason my keyboard sporadically stops working, as if was disconnected, although the Num Lock led shines. Unplug and replug don't help, only reboot.
Driver reinstall didn't help. Replacing the keyboard didn't help.
I've tried to find some pattern - at the beginning thought that it happens after awaking from sleep, but no, I've deactivated sleep mode at all and it happens just in the middle of my work.

Does anyone have an idea?

Thanks in advance!

The problem was solved by restoring my system to the restore point before the two last Win7 updates.