I had the same error, and I found the cure!
You can’t use remote assistance if you use MSN Messenger 6.0, at least, the assistant is forced to use Windows Messenger 4.7 since remote assistance don’t work through MSN Messenger at all, exept that you can request it! Just shut down MSN 6.0 and start Windows Messenger 4.7 … voila, you can now assist another WinXP user!