problem with internet! PLEASE HELP!

Recently I've had a very stange thing happen when I log on to the internet. I have dialup, and about 30 seconds after loging on I get this message... This sysstem is shutting down. Please save all work in progress and log off. Any unsaved changes will be lost. This shutdow was initiated by NT AUTHORITY/system time before shutdown 0:45 Message. The system process 'C:\WINDOWS\system32\lsass.exe' terminated unexpectedly with status code -1073741819. The system will now shutdown and restart. I think my computer has a virus and this is hapening because of it. I don't know what to do about this problem, but I hope someone out in Internet land does. Thanks, Robbie