Windows Server 2003 Rebooting every hour

Since a few days one of my Windows 2003 servers also reboots every hour. I tried to deinstall everything that was not important, stop unnecessary services, but it still reboots every hour.  In fact in 60 minutes and about 30 seconds:
Boot Time      Difference
13:55:31      
12:55:19      1:00:12
11:54:50      1:00:29
10:54:13      1:00:37
9:53:42            1:00:31
8:53:09            1:00:33
7:52:35            1:00:34

The eventlog shows The previous system shutdown at 7:52:35 AM on 1/21/2009 was unexpected.

I checked the BIOS but did not see anything strange. The power management had the setting that after one hour the hard disk could be switched off, but after setting that to Never, it also rebooted.

The only things running on this server (now) are:
"      IIS (With ASP.NET)
"      SQL Server

It is hard to do a virus scan as it does not complete in one hour, but so far I have scanned it in parts (program files, windows/system32), and still continuing to select more parts, but I assume if there were an active worm it would have been detected as part of the memory scan.

The solution mentioned here (manual reboot) does not work for me.