I would just add two more steps, to a great list so far.
6? Check whats running at start-up. You can use the built in Microsoft configuration utility (Start->Run-> msconfig), or a third party tool, like ccleaner already mentioned. Get rid of those things that run automatically that you don't need, such as quicktime, adobe acrobat, instant messengers. They don't have to run at startup, since you only need them running when you are actually using them.
7? Restart the computer. So often, when I get these calls for a slow machine, the user hasn't restarted in 3-4 weeks. Not restarting regularly can leave old processes hanging in memory, using up resources, thereby slowing your system. A restart will clear all of that out.
Just my 2 cents.
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