I reinstalled my windows (7 SP1) five days ago and BSOD has happened many times since.
This morning it happened 3 times in one hour when I was watching a film with Windows Media Player.
Please help me, I'll be very grateful.
I have uninstalled Avast as you suggested and no BSOD happened since.
However, it's bugging me that the crashes seemed to happened so... randomly.
For example, BSOD had happened 3 times in one hour when I was watching a movie (H.264 in a .mkv file) with WMP 12 this morning but after that, my little brother played Battlefield Bad Company (a game with high computer resource demand) for 3 hours without any problem before I came home and uninstalled Avast.