Blue Srceen/Lagging Laptop

Hi i have a Dell inspiron model N5110 with windows 7 home premium 64bit, 4g Ram, Intel core(r)i5 @2.30GHz, 1gig Nvidia Graphics card

Lately my laptop has started to lag with everything it does, YouTube clips are impossible to watch opening folders and programs is slow. Then i tried playing halo combat evolved and it is really laggy wheres when i played it a while back it would run really really smooth with graphics at high. Then it started to get 'Blue Screen' a lot when i opened some things this would happen 5 times in an hour. My brother in law had a look at it and did a few things to it and blue screen doesn't happen as often anymore. i've done a few virus scans and haven't had any issue's except 1 and that was fixed.
So please if anyone Knows what im talking about please reply because i dont want to take it to a shop and have to pay for something i could do myself and i have only had the computer for 3 months.:)