yes, there are two add. trace-files, one PMON.trc saying "error detected in background process" and one LGWR.trc with the same message although a little later in time than the PMON-tracefile.

Last lines in the alertlog are:

Restarting dead background process SNP0
PMON: terminating instance
Instance terminated by PMON, pid = 1468

The win2000 eventlog says "unable to begin another thread"

It looks like a hardware-problem to me but I'm just not sure.