My PC is running extremely slowly.?

Just lately my computer has become extremely slow. I takes a very long time to open files and applications. I have run Adaware, Spybot, CCleaner, AVG Anti Spyware, AVG Anti Spyware, Webroot Window Washer, and CW Shredder. Apart from a few low risk threats nothing else was shown. I have run defrag and disk cleaner, cleared all temp internet files and cookies. Can anyone help as I do not know what to try next?