I now run my Vista machine with Windows update switched off because it was chewing up vast amounts of CPU
I had to do that on an otherwise decent enough Win 7 machine recently.
Re the original question.
I would try creating a new user. Reboot machine and log on as new user. Check performance. If resolved copy/move files from old user to new one.
I have done this a few times with success.
Also check OS integrity with:-
Run the following in an "administrator" command window
sfc /scannow
If unfixable errors are reported you will then need:
Dism /Online /Cleanup-Image /RestoreHealth
redo sfc /scannow
(Obviously? you need to be on the Internet to get the Online bit to work:-)
If errors still reported the OS is corrupt and will need to be re-installed.
Good luck.