Peter has signed up for iTunes Match, but he keeps getting an error message that says "try again later" whenever he starts iTunes in Windows, citing an "unknown error 4001." There's a solution at MasterYourMac.net that says if he turned Match off and back on again, it would fix it. Leo says that's crazy and probably won't do a thing. Apparently if he turns it off with the shift key held down, that will disengage all the Match services. That's an odd fix, but worth a try.
Some music files with the word "error" attached to it may be sticking things up too. Deleting those files may fix it. There's also talk of a bugs in iTunes Match that has been going on since iTunes 10.5. He can also contact iTunes Support and have them fix it on their end Check out this link for more suggestions. Also check out this article from Macworld.