Is it XP home?
Unfortunately, XP home doesn't give you full NTFS control over files, and if you select "use private folders" and then re-install XP, you'll be able to see the folder with all your old files in, but won't have access.
Sounds like your RAR is remembering the security settings of the files within and they're specific to your old username (which may be the same name, but to the computer it's different).
http://www.winxptutor.com/ownership.htm