Since you had 15 FPS, I will guess that you have at least the minimum requirement of 1 GB of dedicated video RAM.
So, don't panic! You have tons of options. First thing I would try to do is to turn the environmental map slider to none.
Next you can move top two graphic detail sliders to the left (no pun intended).
You can adjust the max texture size down from whatever it is. 4096 is the highest definition down to 256 or 512 as the lowest.
Also, the screen resolution can help gain FPS. This will have to do according to the size of your viewing monitor.
* After that you can panic, but only because you will not see the very cool cockpit effects if you turn off post processing, but as I just learned, probably most GVers will be doing that anyhow to avoid the blinding flash when in the scope.
** Even then, only seriously panic if you are unable to upgrade video cards. But back to the obvious, please post your dxdiag.txt as an attachment (helps to take some of the guess work away).
Puller,
Windows has a built in hardware id system. Go to run command and type dxdiag.exe (should do the trick- if not let us know). There will be a button at the bottom that allows you to save the results as a txt file, that you could post here but more helpful to HTC if you also post it in the sticky thread above.