Anti-virus?

[okok]

I am about to start using a script that will let users upload files to the server and download them. I was wondering whether there is some external service that would scan such files for the security of users, or whether there is some other way you know or can think of to detect harmful files (other than just limiting file types to ones that are not likely to contain harmful code). I suppose that installing an anti-virus software on the server that can be called on demand by a script isn't possible. Or is it?