Well, everyone who has this problem getting X-com Ufo & X-com: TFTD to work with Windows 2000 Professional, here is a solution of my own:

Connectix Virtual PC 5.1

First I tried almost every solution mentioned in this forum, but with no success. I didn't manage to get neither of games to work with my OS.

First you download this Virtual PC program from:
http://preview.connectix.com/trial/downloads/VPC_51_Trial.exe
(though this program need the original cd of that OS which you are going to install)

there are simple instructions and mentions that this will be a trial version of this program, but it still works fine with a *****, whic you can download from:
http://www.keygen.us/get.shtml?201724

When you have downloaded this program, you simply work out with the new OS installation as told on the readme (dos, win98, win95..etc) and then you can run two OS as the same time.. (this gets you towards the new ability to play UFO in a separate window!)

both graphics and sound work very well in both games, and I might say.. It took many weeks to find out this method. But now it works. And most important, the game works and I don't have to uninstall windows2000 just to play these wonderful piece of game.

Looking forward playing lots and lots of X-com Ufo & X-com: TFTD.