virus maybe?

I have aDell T700 running windows 98se.  I have Norton SystemWorks 2003 installed with all updated virus definitions. 

What does this mean?  Under virus alerts in Systemwork Antivirus section, it says 11/20/03  autoprotect-Unknown-Access Allowed-Masterboot Record-Write to Masterboot

Source:C/WININSTO.400\suwin.exe

I started up my computer today and the message came up saying something was wrong with the Masterboot.  I clicked on the one that said to ignore and run a vrus check of my computer and to run Liveup date.  I did as it said and nothing showed a virus.  I found what I wrote at the beginning after going through the Systemworks activity log in Antivirus.  Should I be concerned?  Could a virus be on the computer?  Any info would be great. Thanks