Okay, so i bought Fo3 sometime ago, and it worked fine on my pc, Then in crash (the computer) and i had to get some new stuff for it,
Then i install the game again, and everything is normal, it sets the settings to normal, Okay, but when i press Play, the box just disappers and nothing happends... happening to anyone else? Please help meh! im a huge Fo3 fan, and i really wanna play it...again

Post a dxdiag.
XP: Go to Start->run, type “dxdiag” and hit enter. Click the Save All Information button near the bottom right. This will save a Dxdiag.txt in C:\ by default. Then open the dxdiag.txt and copy/paste the information into your post.

VISTA/Windows 7: Go to Start->Search, type “dxdiag” and hit enter, click the Save All Information button near the bottom right. This will save a Dxdiag.txt in C:\ by default. Then open the dxdiag.txt and copy/paste the information into your post.

I have had the same problem too, even with my computer runs the game well. For me, simply rebooting computer fixed it, or some of your mods is crashing. Turn out all your mods, only leave Fallout3.esm on and try if it works.