PC shuts down

I’m working on E-machines PC with Celeron, 128 MB of RAM, Win XP Home that shuts down by itself after 10-15 min. Message box with counter pops-up, starts counting from 60, and shuts down after 60 seconds. This is the message:
“This system is shutting down. Please save all work in progress and log off. Any unsaved changes will be lost. This shutdown was initiated by NT AUTHORITY\SYSTEM.
Time before shutdown: 00:00:60
Message:
Windows must now restart because the Remote Procedure Call (RPC) service terminated unexpectedly.”
Remote Assistance checkbox was checked and I unchecked it, but still the same. Scanned computer for viruses, and didn’t find any.
Has anyone encountered this problem?
Any help would be appreciated.