I have high speed, but the last couple of weeks my modem has been really slow. Any suggestions?

I have a wireless modem, through clearwire if that helps, and for the last year or two haven't had any problems. But recently it has run really slow, like when I had dialup. I run spybot search and destroy and have AVG virus protection that updates daily. I drefrag and do disk cleanup and that doesn't seem to help. Any suggestions would be appreciated.