i recently ran DCDiag.exe and one of the errors was
failed test frsevent and failed test kccevent
would this prevent a Windows XP prof client computer from joining a domain?
Depending on the exact error it could. Specifically the kccevent. I can't think of a FRS error that would cause it, but it would cause your clients to get incorrect/outdated policies and scripts once joined. KCC is related to authentication so it's possible.