Hey guys, I'm also using WinXP but I haven't had any problems, so here are some suggestions you may try.
1. Make sure you have XP updated, this may seem like a pain in the butt, however 90% of the problems with games I have ever had with XP have been related to the fact that I didn't have all the latest updates.
2. Get the most recent patch for the game, this is really important.
3. Install the game, then install the patch. Now BEFORE you install any mods or anything, go to the Start menu > to the listing of your game. If your trying to run EAW, click on "Configure Starfleet Command II", if your trying to run SFC:OP, click on "Setup Utility".
After you configure those, then try and run the game, and enter your cd-key. If successfull, you should now see the main menu. If you want to play the stock game, have a ball, you're all set. If you want to install mods, exit the game.
4. Install any mods you want now and you shouldn't have anymore problems. One more suggestion though, install one mod at a time, and then try to play, that way if a certain mod gives you any problems, you will be able to tell when you start up the game.
Thats about all the help I can muster from memory at this time, if I think of anything else I'll post back later.
Hope this helps.
GateKeeper aka Jim