laptop is get slower and very difficult to use

i have a hp pavilion g6 2027tx notebook laptop.i purchase it 16 months ago.it has a problem now.it is very slower now and difficult to do normal works.even watching a movie.it stuck and playing when we see a video.i thought is was a virus issue and install windows 7 again.but there is no improove.Although i can see movies and same videos through my portble hard disk with no interuptions.but bios diagnostics and others are showing my hard disk is healthy.i cant diagnose the problem.please help me.