First off , if the game files arent on your computer , then your right , you wont be able to play it.
But double check ... if you installed steam to their default location. Then Fallout 3 will be located here.
C:\Program files (x86)\Steam\Steam apps\ fallout 3 .... verify you have a fallout 3 folder around 6 gigs ...
If so come on back and post a dxdiag of your system hardware , so we can give more specific advice on fixes.
You do this by bringing up your command prompt ... Start/All programs/Assecories/command prompt .... works
Then type "dxdiag" wait a sec ... then youll see a window with button that says "Save all Information" click that and select the destination to save (Desk top is easy) Open that file up and copy paste in these forums.