You'll only be able to uninstall it if you selected the option to backup the previous service pack when you installed SP6a. The problem with that is if you have installed SP6a more than once, or if you went from SP4 to SP5 to SP6 to SP6a, it will only recover back to the last service pack that you ticked the 'backup' box on when installing. If you run a recovery/repair of NT, either from a repair disk, or by selecting the option from the NT install cd, it will take the PC back to service pack 1, but you'll probably find you'll need to reinstall lots of drivers and set up things like networking again, so this is definitely a last resort.
Adrian