yoyo - I think you hit the nail on the head - a system restore point will eventually disappear be it because it is 90 days old, your system is out of allocated hdd space for restore points, you manually delete it to gain hdd space, or you disable system restore for whatever reason. Thus you still have the option of "getting rid" of Sp1 via add/remove programs.
BTW -my 1st restore point was 771 MB - nothing to sneeze at!
hipster here is the answer to your question about why the backup using add/remove.
Does System Restore uninstall my program if I restore to a point before the program was installed?
Answer: No, System Restore does not completely uninstall any program by itself if restoring to a point prior to the program installation. Since system restore is based on an inclusionary model, any file added or modified by the installation which System Restore does not monitor or files added to or modified in a non monitored drive will not be tracked by System Restore. Thus System Restore cannot delete or undo any modifications made to these files. To remove all the changes an installation may have made on the system, the user should first use the Add/Remove option in the control panel to remove the application prior to using System Restore. System Restore however will undo all recorded changes made to the registry & monitored files caused by the application install including:
Delete monitored files the program installation may have added to the system
Undo modifications to monitored files made by the installation
Replace the current registry with the registry snapshot taken when the restore point chosen was created. (*note: some current values will persist)
Any file type not monitored by System Restore added or modified on the system will not be restored or removed. E.g. .jpg, .txt files.
Sorry for the post being so long - but hopefully this will help explain at least part of what is going on.