High Spec laptop freezing for seconds or two

Registration date
Saturday May 11, 2013
Last seen
May 11, 2013
My laptop (recently bought 2nd hand came by post so kinda gutted) i5 2nd gen, 4 gb ram , 640 hdd, 1 gb nvidia gpu.

problem is it freezes for second or 2 contiguously every minutes. suppose im gonna type dan it will take time , playing music it freezes for second or two n carry on in every 10-20 seconds.. i checked ram.. its fine .. i reformat and installed new os.. checked with all anti virus cleaner and malware softs .. done registry cleaning ..
CPU act normal when i checked with speedfan, process checker and all other utilities..
couldnt find any problem..

what would be the possible problem??