I know that every company has budget constraints, but it always amazes and amuses me when people are looking for "cheap" backup software for a business server. Please understand, I'm not trying to be a smart-ass, but "cheap" backup software is what you are using - the backup that is built into Win2K Server and has a lot of reliability issues. You are taking a very narrow view of things if you are thinking in terms of software that will work "for single-file restoration." When you are looking at what backup software costs, I always recommend that you estimate what it would cost your company to try to recreate all of the data that is being stored on that server in case of a full system crash.
With that in mind, I usually recommend Veritas Backup Exec as a backup solution. It is reliable, fairly easy to install and learn how to use, and fairly sophisticated, and does everything from single-file restoration to restoring the system state, registry hives and Active Directory. For the basic backup product that will back up one server, I believe it costs around $1,000 (full retail - you may be able to get it for less through on-line vendors like CDW). If you have more than one server and/or need additional options to back up open files, SQL or Exchange, etc., there are add-ons for that, too.
Hope this helps!