Laggy and choppy video playback; Windows runs very slow

All was well with my computer and one fine day video playback became very laggy and choppy. Also the computer in general slowed down considerably. My first thought was that there had been a virus attack and hence I ran a thorough scan. Nothing showed up. So formatted the whole system and installed Windows 8. Nothing changed. Videos continue to be laggy and overall system performance is slow. I read somewhere this could be because of high DCP latency caused by faulty drivers. I used the original Windows 7 backup DVD given to me by my laptop manufacturer to restore to factory default. No change - laggy video with slow system performance. I used the DCP latency checker to figure out what was causing the high values I was getting and pinned it down to the WiFi driver. The latency is down but choppy video playback and super slow OS continue to bother me. I've run out of options and my patience it's wearing thin. I am very close to smashing my laptop into bits because of the frustration it is causing me. So I beg you to please help me. What is f*****g wrong with my laptop and want can I do to bring it back to its former glory.

Laptop configuration -
Intel 2nd gen Core i7
6 GB RAM
640 GB HDD
ATI Radeon 6770M
Blu-ray Drive
15.6" display