cleansing
Most anti-virus programs have a system clean-up feature, which might include de-fragmenting the disk, removing temp files, and cleaning up the registry.
The registry tends to get filled with entries that don't do anything. But it still has to try and process them. There are those who will tell you to not f**k with it, but the older a computer gets the more plaque builds up. At least that's the way I understand it.
I've used Ccleaner, Norton, and AVG to clean my registry. I have seen the best results using the free version of AVG, which really helped me clean out a nasty infection recently that prevented a windows update from successfully installing.