There are a few programs like ccleaner, errorexpert, etc ... with an uninstall feature.
http://www.google.com/webhp?hl=eN#hl=eN&q=force+uninstall&aq=f&aqi=&aql=&oq=&gs_rfai=&fp=ddaad785462b6469
Or
Do it the old skool way. Go to target directory: C:\Program Files\BlackIsle\Fallout2\
Highlight all the files & folders and nuke'm... huh... I mean press del.
Don't forget Vista has a nasty habit of splitting up directories. That's why I recommend not to install in the Program Files folder. Don't ask me why it's a Vista & Win7 thingy.
So also nuke all the files in (Vista/Win7) C:\Users\"yourusername"\AppData\Local\VirtualStore\Program Files\BlackIsle\Fallout2
You can delete the folders: BlackIsle and Fallout2 - if you're going to install FO2 somewhere else next time.
It's probably a good idea to use http://www.piriform.com/ccleaner afterwards to perform a general cleanup & register scan.
Et voila FO2 is uninstalled.
*Edit* I forgot to mention: if you're going to perform a full manual install next time you might want to delete BlackIsle folder in
C:\Users\"yourusername"\AppData\Roaming\Microsoft\Windows\Start Menu\Programs\BlackIsle
as well because you don't need this folder in your Start Menu. It only clutters up your PC. Just make a shortcut to fallout2.exe on your desktop.