Would uh... really help to know the version of Windows it's running.
Default suggestion if Vista or higher: Burn an ISO of the windows repair tool for that version of windows, use it to replace the windows core files. This should allow you to boot into safe mode, enable the netadmin account, log into that via safe mode, and delete any infected files that can't be deleted otherwise.
However, the idea of using an operating system that's already been infected is just... not good. You should wipe the disk and start over.