Computer freezes after key press

Recently, due to some minor boot problems, I formatted my computer and reinstalled the OS (Windowns XP SP2) using a different CD (the first one was SP1).
After installing/updating some drivers and softwares (don’t remember which ones) the problem started: during/after the boot, if I press any key on the keyboard the mouse and keyboard freezes completely. However, this don’t happen all the time.

I already tried to turn off all the startup softwares, tried to update even more the drivers and used the Windows Update to see if it fix the problem. No success.