Thank you for suggesting me to delete the corrupt files. But before i read your message my system was dead, means not booting up. it hanged in start up repairs for many hours, so finally i formatted the drive and installed win 7 32 bit version. Actually i was facing problem from last whole month, i used to install win 7 64 bit and in two or three days whenever i used to ran an update or install some software or driver, it used to get hanged and i ended up doing installing fresh, each time finding some other new reason.
i hope for now that shifting to 32 bit version solves my current problam of reformatting in few days!!!
both the windows version were genuine copy.