As best as I have tried, P2 won't work under XP, at least on my system--but that could be because of my ATI video card. If you wanna try it on yours, download the P2 for Windows patch from the CIC files, make a shortcut on your XP desktop to DARK.EXE, right-click on the new icon and select Properties from the menu. Click the 'Program' tab, then 'Advanced' button, then check the box next to 'Prevent MS-DOS programs from detecting Windows.'
If you are able to get it to work this way, make sure you also copy SETS.IFF from the P2 disc into your C:\DARK directory, or you may have hang-ups trying to land at certain bases.