You can not use / install any other GFX adapters on Virtual PC virtual machines than included, emulated S3 Trio
. When integration features are installed and enabled, a Virtual PC vm like XP Mode can by default only show 16 bit colors. To use 32 bit, you have to disable integration features.
Some tweaking makes 24 bit possible also when integration is enabled: I got XP mode to display 24-bit color with integration!