Defrag is only part of speeding up, what do you do to cleen?
All computers will slow with the more programs you install, especially if they all want to load at the statup. also a lot of rubbish gets put in the registry and parts left behind, doing nothing and going nowhere. A program such as the free CCleaner and a registry cleaner program will work wonders Also removing programs that you no longer use or want will help.
Sorry you will have to start from scratch if you reinstall windows, as if you think you will require more knowledge to run progs suggested then making a disk with all your progs etc. will be too complicated to explain