This is not a question but a statement of case study on fixing a bad installation of SP3 hopefully the time I spent can now help others...
I have just spent the last 4 hours trouble shooting a bad installation of sp3 on my xp pro machine. My USB peripherals were causing BSOD 0x000000FC and/or 0x0000007E . I could not boot into normal or safe mode. I first went to Dell support and they helped me trouble shoot that my keyboard and mouse in the USB's was the problem. They then referred me to the link below. Microsoft is offering FREE sp3 tech support. I chatted for about 120 min and we uninstalled and then reinstalled SP3.
Helpful things to know.
#1. To easily uninstall SP3 use the XP system CD to boot into the recovery console.
#2. type the following: cd $ntservicepackuninstall$\s
puninst and hit enter.
#3. type: batch spuninst.txt you will see some files being copied, some files unable to be accessed, etc. all this is normal.
#4 type: exit and press enter and you should have reverted back to your previous service pack.
To fix the issue for me (your issue may be different) I had to actually uninstall both Kaspersky and Spy Sweeper and then go to the MS site and download the sp3 as a full service pack INSTEAD of going to windows update and d/ling it there. The full pack was a much larger file. I'm not sure exactly which of the things, removing anti-virus and spyware or the other full version of sp3 fixed my problem. I'm pretty sure I could have just disabled the anti-virus and spyware protection but who knows. The take home point here is Free SP3 support at the above link from Microsoft! :-)