Corrupt file

Hello, I had a trogan virus and it crashed my comp. When I reboot my comp. I get this black screen with this message. Windows could not start because the following file is missing or corrupt: system\drivers\isapnp.sys. You can attempt to repair this file by starting windows setup using the original setup cd-rom. select 'r' at the first screen to start repair. I got to the repair screen an it asked. Which windows installation would you like to log onto (to cancel, press ENTER) ? I do not know what to type in at this point.????