I would suggest doing a START/RUN/MSCONFIG, go to the Startup tab and uncheck anything that you don't want to run on startup (you can always run it later) or looks suspicious. All I have is Systray, Norton anti-virus and ZoneAlarm on my Startup.
I would suspect something runs on startup and chews alot of CPU cycles and disk I/O.
OldEconomy buyer beat me to it. Do what he says . . . =:-)
Clean the startup, by running msconfig as described by jeffc in #7. You probably have all kinds of programs that start when you start up your computer, and there is no need to have most of them running at all. Use
THIS LIST as a reference if you are not sure which programs you don't want to run at startup. It is the most comprehensive list I have found.
Also, you mention temp files. Do take some time to get rid of your temp files.
Even if these suggestions don't fix all of your problems, you should find them to be beneficial anyway and they should be part of your regular computer maintenence.