Windows logon dialog box pops up, even in Safe Mode

I am running a 5 year old Dell Dimension P4 PC at home and just recently I restarted the computer and now it prompts me for a UN/PW to log onto Windows. I have never set up either on this machine and have no idea how to log on. To make it worse, I get prompted to supply a UN/PW even in Safe Mode. I can’t even switch to OS install mode to reinstall Windows with my install disk. I’m sure this is some kind of virus that’s causing this, but I need to bypass the logon so I can go into the user accounts to set/reset the UN/PW. Please assist!!!