There`s a method of hooking the directx 9 dll in gta:vc and creating a speedometer overlay on the screen done by a guy named spooky
here.
This method could intresting for hooking into the game but in xp sp2 has this new memory protection, so im not sure how easy it would be (basically all it does is unables anything to write into a remote process other than it`s own, microsoft attempt at stopping buffer overrun`s). One idea i thought about is to change the hWnd, hThread of the inject process to the same as the process you want to hook into then inject quickly and exit (hopefully confuse windows). im not sure if LoadLibraryA method would work with xp sp2`s new memory protection.