I don't know a lot about computer but isn't it possible yours isn't capable of playing them?
My computer is truly brilliant but it has a reaaaally crappy graphic card so I can't play games on it.
Have you thought about upgrading yours or getting a new one completely?
I do know alot of computers, mine is handbuilt and overperforms for its size & it isnt overclocked. Its only drawback is the HDD transfer access speeds and they are normal. On Windows (useless) benchmark I am 7.5 out of 7.9 on all but HDD access which is a respectable 5. Also I have previously played Fallout 3 on this machine. Additionally I play more demanding games on higher settings.
Its really more a problem of code and ini than hardware. I dont really wanna downgrade back to XP tamper with a whole bunch of dx files and jury rig working sound files back into FO3 just to make it work. Bear in mind FO3 was released when Win 7 & Vista showed up so its hardly credible that the designers couldnt account for those OS's vagaries.
Concerning New Vegas, if they are using the same engine the same problems will appear. Common sense is fabolous isnt it

If A works in X & B=A, and A is superceded by W=A(& therefore B), then (X=W)=(A=B). Insert WP, Win7, FO3 & FONV into A,B,X,W where it makes the most sense for you.