It's running Windows Server 2k3. Not allowed to go in to real details though.
Anyway, once or twice a day the CPU runs at 100%, but I have never been around when it happens. For instance, it can take up to 15 min to print an A4 page, and Process Explorer and Process Monitor will use 50-90% of the CPU thus it cannot be used to determine what is causing it. There are no scans, backups or anything of the sort scheduled either so we are really confused as to what is causing it.
/Patrik