My computer is stuck in setup!

I was using Win XP Pro SP1a with no problems. I downloaded and installed all
updates from the Microsoft site with no problems. I did a disk cleanup,
system file check, scan & immunize with several different antispyware
applications, scan with antivirus, and defrag. My system ran wonderfully. I
received SP2 on CD from Microsoft and installed. BIG problems.

My desktop (with icons) appears, but none of my programs (such as defrag,
internet connection, etc.) will run. I tried to uninstall SP2, but received
a message saying "no". I tried to re-install Win XP Pro SP1a in a clean
boot, and now my computer is stuck in setup. A message appears saying,
"Fatal error: Setup failed to install product catalogs." (Other messages
say, signature for win XP upgrade is invalid, cannot find text for message
#0xfffffdda in message file for syssetup.dll, and that the CPU setting is
incorrect.) I set the BIOS (including the CPU setting) to DEFAULT, with no
improvement.

I tried to restart in Safe Mode without success (but it did tell me that I
am running SP1). Right now, my computer hangs in setup, restarts, hangs in
setup, restarts, with all the messages, above. The rescue disks didn't
work--does anyone have any ideas?