Do you have, xp, vista windows 7 ?http://www.nvidia.com/Download/index.aspx?lang=en-us
be sure to select the right card (not the 400m series but the 400)
select the proper os.
What video card did you have previously ?
Um the title is this " Windows 7
wont detect my gts450 hardware. So W7 I guess.
What card did you have before ? If you had a card before and not just onboard did you remove the old drivers first before removing the card ?
What you should be doing is downloading the drivers before installing the new card and then uninstalling the old card by removing the drivers from the system and then taking the card out (with the power off obviously).
Install the new card then run the driver instalation package.
Probably best if you run driver sweeper (free download) now though as there may well be some conflict issues somewhere.