HELP PLEASE! can't f8 format

I have recently been hit by a pretty nasty virus/malware app and would like to do a factory restore. The problem I am having is that when I select the Repair option per F8 on startup I get to the logon screen and all it shows is "other user" with the username/pass fields blank. I have tried every possible user and user/pass combo I could find and think of. Every time I input a value into these fields and try to login I get the msg "The specified domain either does not exist or could not be contacted", if I try to logon without putting info in the field I get a windows security msg saying I must have a user/pass to logon. I looked into every possible aspect of user accnts, changed the need to logon, removed all passes, gave full control to every part of windows and still I get the same screen. I need to know if there is a way to bypass this or if the virus may have changed the logon setting for the repair operation?