If you install a service pack that renders your computer unbootable, and you did NOT prevent the default uninstall archive from being created, you would normally boot to Safe mode and use the Add or Remove Programs applet in Control Panel to uninstall the the service pack.
If the install also prevents booting to safe mode, try the following:
1. Insert the Windows Operating System CD-ROM into the CD-ROM drive.
2. Restart your computer.
3. Boot the CD-ROM, pressing R on the Welcome screen, or start the Recovery Console from the menu.
4. Select the Operating System instance that you wish to access and enter the Administrator's password.
5. Change (CD) to the archives spuninst folder, normally located at C:\WINDOWS\$NtServicePackUninstall$\spuninst.
6. Type batch spuninst.bat and press Enter.
7. After the batch command finishes, type Exit and press Enter.
8. Boot to Safe mode, by pressing F8 when the boot menu appears.
9. Use the Add or Remove Programs applet in Control Panel to uninstall the the service pack. This step is necessary to rollback the registry changes that the service pack made, as well as removing the archive.