800MHz and WinXP. wow!
I know this won't help you, but think about Win2000 or Win98SE
But back to your problem: We have all three X-Com games (okay the first is named UFO. Who cares?) And all those 3 version run under DosBOX perfectly.
So I think you have not a version from this sit. Am I right?????
Your problem is that, if you got a originally windowsversion your pc is much to fast. Anyway what you do, you can't change this matter of fact. Those programms which slow down your CPU are... well... let me say not so good for the life durance of your computer.
My advice is that you should download the version here on this site
http://www.abandonia.com/genre.php?search=x-com
Enjoy it
