So no, there's definitely no "Delete software restriction policies" in the group policy editor in Windows XP like there is in Windows 7.

What you would have to do is just go back and change things to the way they were when you first created the default policy. Set the Security Level default back to "unrestricted" and those defaults do nothing.

Or delete:

HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\Safer

and there is one more thing

While that will defang anything you have done with software restriction policies (on a local computer), your Software Restriction Policies will still show up in gpedit until you delete:

C:\WINDOWS\system32\GroupPolicy\Machine\Registry.pol

Be warned that that file is for all the group policies that may have been created, not just software restriction policies. So deleting that would take you back to Windows defaults.

P.S. I mean to summarize: Deleting the Safer registry subkey and the Registry.pol file resets that stuff and it will say "No Software Restriction Policies Defined". If you add new policies, it gets created again.