Please follow these really simple steps:
1) Open up your fallout 3 directory.
2) Download a re-texture from Gamebanana or from fallout nexus and open up the archive.
3) Drag and drop the texture folder to your fallout 3 data directory.
4) If it asks you to copy and replace do it.
5) Now go to your "Whatever driver you have C or D or F":\Users\user\Documents\My Games\Fallout3
6) Open up your FALLOUT.ini with notepad
7) Find the line-command "bInvalidateOlderfiles=0" and change it to "bInvalidateOlderfiles=1"
8) Click save and close it.
9) Run the game and have fun!!!