sounds like you had a nasty infection that pretty much destroyed your windows installation, it might have been the "anti" virus you downloaded. malware likes to pretend to be a AV program, and googling for "free anti-virus software" is like opening a can of worms unless you know what to look for.
At the "enter the name of the command interpreter" prompt, you should be able to enter C:\windows\command\command.com to get a usable prompt.
Either way, you are certainly looking at a complete reinstall to get things working again.