Hi CoCo, welcome to NST.
Did you install SP1 through Windows Update ?
There's a known problem with an apparent hang (frozen mouse/keyboard, blank screen) at 99% completed.(the HDD LED flickering indicates that updating is still continuing)
If you hit reset too soon (give the system 30 minutes for the HDD activity to stop), you'll break the SP1 install.
If this is your situation, try F8 at power-up to get into safe mode, and use system restore to take you back to the checkpoint before SP1.
If you can't get into safe mode, boot the Vista DVD, "repair your computer", then access system restore from the next screen.
If this successfully gets you back, make sure you give SP1 much more time next time. (have a coffee while you wait)