It's basically the operating system. For various reasons windows get slower and slower over time.
there's two things you can try. download a program like 'tune up utlities'
http://www.tune-up.com/ - 30 day free trial - and run the 1 click optimizer. if used this program for years and it really does help.
the other more effective but more drastic method is back everything up, format your disk and reinstall windows and all your programs.