1. Make sure there's enough RAM.
2. Make sure there's enough RAM.
3. Make sure......
You get the gist.
After that it's mainly a matter of uninstalling unwanted programs, especially those that run in the background.
Also, try a different antivirus product. Several (especially the "big boys") are known to go "bad" and really slow things down. They can do that anyway even when working properly as they have far too many functions in for most users.
Look at indexing - turn off and remove anything like Google desktop or the Microsoft equivalent. And turn off indexing on the drives too unless you use "Search" all the time (it only makes asmall difference in speed anyway).
Registry cleanup is a good idea too.
Defrag as well I guess, but not a priority usually.