Reinstalling Windows is neccessary from Time to Time.
I just did it about a month ago, course it was that slow e.g. it took about 10 minutes to be fully loaded at startup.
Cleaning the registry and defrag the system i do every once in a while, also got FW and antivirus programm running and up to date, so i didn't see a real reason for Windows beeing that lame.
In addition to this i had to remove this ugly SecuRom which came with FC2, the game was shit anyway and i never ever play it again (and i just keep away from games useing this "protection" from nowon).
At the end i decided to reinstall the OS and now it works amazing fast again.
Of course i reinstalled windows before, so i seperated my HDD into partitions for Windows/Programms, Games and Data.
This way i just need to delete the Windows partition and of course reinstall Programms and some games, but i keep all of my wonderfull data.
Sure it's a time consuming thing to Formate your OS drive and reinstall OS and all programms, but after a few times you'll get used to it.