Missing or corrupt file(s)

A few months ago I had my computer redone. Now as time went by, I have a lot of problems downloading and installing programs, even programs that I could easily download before. The error message reads that “MS-DOS and Windows are not suitable for running together.” Someone else did the work for me and I don’t have the particular program myself “Windows 2000.” Everything I have found has told me that I have a corrupt or missing file, the Autoexec.nt, Config.nt, and/or the Command.com. Is there a place I can go to download only the file(s) I need without having to buy the whole program? Obviously, I learned my lesson about some people working on my computer.