Is there a registry tweek to tell Windows Setup to start over? It tries to continue where it left off after I reboot. I can get to the Recovery Console.

I tried to go from SP2->SP3. It failed, and I couldn't get to Safe Mode. I put in the SP1 disk (I don't have an SP2 boot disk, just the upgrade package.) It ran a couple minutes then gave me the error "the signature for windows xp professional upgrade is invalid" error code 800b0100. I rebooted to Safe Mode. I get to Safe Mode then say "Window XP Setup can't run under Safe Mode" press ok to restart. I put the SP3 disk back in, trying to get the "repair" option I didn't ever see putting in the SP1 disk, and it tried to continue the SP1 install - on the 4th step, and then gave the same signature error above.

I need to get it to start over, so I can get to the repair option, to go back to SP2 (or install SP1 then add SP2 to it). Is there a registry tweek to tell Windows Setup to start over?