alienware m15x laptop that can't play games on the lowest graphics settings...

Ok so my alienware m15x laptop is about 4 months old.

Processor is an intel core i7 q740  @ 1.73 GHz

6 gigs of ram

64 bit operating system

ATI mobility radeon 5800 series video card

The game i have been playing is league of legend. Which is an online game. But my ping is great and i have great internet connection. Never had problems with it before

Awhile back i got a virus and had no virus protection (which was dumb!)

I wiped my computer with the windows 7 install disk that came with the laptop. I reinstalled the drivers. And now while im playing league of legends, it freezes up, lags, sketchy movement. And it isnt realy playable on the highest graphics setting like it was before memory wipe. It's not realy even playable on the lowest graphics settings.... Anybody have any ideas that could help?

thanks,

taylor