by "buyed" do you mean you didnt? Also, going to need more hardware specs than the brand names. Things like amount of RAM, Gfx card are important. Also, fallout 3 is known to have that bug and has a number of "fixes" including editing a .ini file and inserting a line which allows it to properly utilise the number of cores you have...but without knowing what CPU you have we cant know how many cores it has.
Try this forum for possible fixes though: http://www.sevenforums.com/gaming/20199-fallout-3-windows-7-w-quad-core.html