It's one of the worms.
This is a very old worm that XP was patched against a very long time ago. Are you running an unpatched OS?
Here's an article about how to deal with MS-Blaster. http://www.trendmicro.com/vinfo/virusencyclo/default5.asp?VName=WORM_MSBLAST.B
OS: Windows XP
I currently have Ad-Aware SE Personal installed, and every time I run a system scan, after a while an error message appears which reads;
This system is shutting down. Please save all work in progress. This shutdown was initiated by NT AUTHORITY/SYSTEM.
Then something about a Remote Procedure Call (RPC).