Many people have decent Windows machines, say 1-2 years old, that are running quite slow. After cleaning off any spyware/adware, removing old and/or unused programs, and disabling unnecessary services/applications from starting up automatically, the machines still run sluggish. Now, we are not talking about machines that need massive hardware upgrades or are being used for hardcore applications – often, just some additional RAM would be nice, but is not absolutely necessary.
So, what is the common thread? McAfee VirusScan. VirusScan seems this comes preinstalled on many machines, and most people just pay for it after the trial period. That makes sense, as it worked for a few months and looking for alternative anti-virus products is a chore. However, it appears that McAfee VirusScan grows more and more bloated over the course of the year or two of its life cycle on the machine, and the machine just cannot keep up. Even turning off much of VirusScan’s usefulness helps little at this point, it must be totally disabled to remedy the situation. (Reminds me of Java! Ha! 
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The usual fix, remove McAfee VirusScan and install CA EZ Antivirus. Despite not being the top ranked, it works for us and is quite fast. Plus, there is a free 1 year trial for Microsoft users.
(If McAfee has any ideas, please let us know.)