Getting intermittant BSODs

I have a WIndows NT4 which has recently been having intermittant(4 times in the last week)BSODs while ‘just sitting there’. Re-booting brings the system back up but what can I do to troubleshoot? I have done a virus scan via a network PC (using latest dat files) and found nothing. The event log references the MINDA virus in the text of the EXTRA.DAT file (I’ve found the same file in several network PCs). No other Nimba/Minda symptoms remaining. Could this be defective ram (512MB), corrupted sytem files?