Find how a server was hacked and advise or put solution in place to prevent re-occurrence

We have a remote server that hosts all of our clients websites. we have discovered that it has been used for file sharing for mp3 music files. Someone has hacked into the server and uploaded hundreds of these files for sharing purposes. We need someone who can 1. find how this happened and 2. advise us on how to prevent such an incident happening in the future. The server is running Windows Server 2003 with IIS 6.0 If you can bid for 2 to 3 hours work and we can see how it goes from there. If you have experience with SQL Server security it would be a bonus. To bid for this project you should have a high rating on GAF and extensive experience with server security.