XP suddenly slow starting after service pack 3 installed

In response to all the media hype about conficker I decided to install
updates to my XP Media Centre SP2. When I went to do so it recommended I
install Service Pack 3. So I accepted and installed SP3. Several updates
were subsequently installed. I made no other changes to my computer and shut
it down. The next day when I turned it on and logged-in it took
significantly longer to complete booting than before. Also it is now running
slower overall. There is heavy file I/O occurring as I can now hear my
hard-drive working furiously while starting-up and often while running
various apps....much moreso than before. Booting before took maybe 1-2
minutes. Now take 5+ minutes. Any ideas. I see on MS's site there's the
ability to uninstall SP3...has anyone tried this with success? Thanks in
advance.