What files are safe to delete?

We are running a Windows NT Server 4.0 SP6a. What I was wondering is what files are safe to delete from the server? We are in need of some more space, but won’t be upgrading the hard disk for quite some time. But I do know that there are alot of files that are not needed that can be deleted but I need to know which ones are ok to delete. Any information would be greatly appreciated. TIA.