faulty GPOs

After rolling out a hundred odd W2K pro workstations during the summer and doing a hurried job of the GPOs (yep … I already know that’s a bad idea!) I have now found that some machines have altered what part of the policy they apply. No changes onthe GPO have been made other than changing of address of proxy server etc.
A newly installed box in the same OU pulls it all in fine tho. How can I delete applied GPOs on the machine (will not accept a new version from server) without doing a whole rebuild?