You can get X-Com running on XP by using VDMSound (a very nice hardware emulator for these old games) -
http://ntvdm.cjb.net/ (as far as I know, its not designed to run on 98).
BUT... its still not perfect. The game is just not made for these super-fast CPUs we have today. From what I can tell, there is no software timer to control the speed, so the game will run WAY too fast, unless you have a P133 or something. There are programs to limit CPU speed, but I find that they only make things even worse (ie: lockups, etc.)...
With 98, you can try to install a DOS driver for your sound card, and boot to DOS. That should work too...