hi, welcome to the board. Try this before you format and destroy all your data. put your windows XP cd in the drive and boot the pc, hold F8 while the pc boots and go into the repair console. type in sfc /scannow then follow the on screen dialogue from there, reboot when done it should boot up at least and give you chance to get your data backed up (if you need to, it may fix it perfectly) hope this helps. by the way, did you select the option to be able to uninstall SP1 when you installed it? hope so, remove it in add remove and try again, or try another SP1 file, the file should be 133 MB (140,440,152 bytes) in size in its exe format, and file version 1.16.121.0 also did you check Here ?