If you're trying to get DF2 to run, right click it's icon, go to 'Properties', then select 'Compatibility Mode' and set it for Compatibility with Windows 98 or 2000.
Indeed you can run Dark Forces on Win XP. There was an article about it on DF-21, but it seem to be down temporarily. I suggest you vist the forums at www.df-21.net and ask there.