Grinding instead of sounds, computer restarts itself

I have been having this problem for the last couple of
days. My speakers randomly switch into playing a terrible
loud grinding, scratching noise instead of whatever is
really supposed to be coming out of the speakers (music and
other sounds), so I have to mute the speakers, but then it
will randomly be ok a few minutes later when I un-mute
them. Does anybody know what might be causing this?

My computer is also randomly restarting itself. It seems
to happen when I have AIM and Winamp running as well as my
browser, and especially happens when I skip several songs
in Winamp.

Could all this be cause by a virus? Adware? Is my
computer just broken?? Email me if you have any ideas.
Thanks you so much for your help!

Specs: Dell Dimension 8100 from 2001 with 4.16 GB free
harddrive space left, and 256 MB RAM. Intel Pentium 4.
Service pack 2 installed. Sound card is Santa Cruz Turtle
Beach. Let me know if you need to know anything else.