Hello Brustlhr

Welcome to PCHF
Try unplugging your hard drive and booting then (with it unplugged). From the sounds of it, you may have a failing hard drive. I am betting when you unplug the hard drive, it will boot past where it got to, but will then say "no boot device" or something like that. Possibly plugging it back in may help.
Actually...
I would feel correct if I knew for sure that your hard drive was in the primary channel. What you really should do is unplug your primary IDE cable from the motherboard. The primary cable is the one closer to the
CPU (the one further away from the edge of the motherboard). Usually the hard drive is on the primary channel. Just do what I said in the first paragraph if you find the hard drive is in fact on the primary IDE cable. ...
If unplugging the devices from the primary IDE channel doesn't work, try plugging them back in and then unplugging the second IDE channel and booting. I'm betting a device is failing, and preventing your computer from booting.
This problem isn't a symptom of your SP2 installation. If there was a problem with that, you would see something happen when Windows is loading.
Let's go from there
