Mouse, keyboard stops responding on xp

PROBLEM: A Dell Optiplex computer is running Winddows Xp Professional. The mouse and the keyboard (both ps/2) stop responding after given time. The rest of the system keeps running fine. Because my system is acpi system, it is possible shut down cleanly with the off button.

Already tried: – disabling acpi pc and installing it as a Standard Pc. It didn’t work

Why is this and what can be done about it.