Use a utility like ccleaner to clean up temp files and old useless reg entries.
Use msconfig to clean up your startup.
Remove unwanted apps.
Ram is almost free, doesn't really "solve" the problem but it does help.
Re-installing an OS that often is silly, doing that just indicates a lack of knowledge :/ .
Hard drive defrags are not going to help much, it's a good practice but people throw it around as a solution far too often. At best you'll get a slightly tangible speed increase, but I wouldn't even expect that. They teach you to tell people this in tech support because it takes a long time and gets you off the phone, not because it really works well.
If you don't NEED windows, this isn't actually too bad of an idea. Although it will be a bit of a learning experience, probably more than you want.
The biggest thing is cleaning up old/unused applications. I'm assuming that your PC (like most) has a ton of random utilities chugging along doing nothing 24/7 (scanner software and the like).