I don't really know what is the problem if you have installed the latest patch for FS2 and also updated W2k with service pack (latest is 2) and latest compatibility updates. FS2 runs smoothly in my W2k, actually you should try next trick:
Open notepad and create a txt file on a same folder where fs2.exe is located and write next lines on it:
start /high FS2.exe
then save and close, now rename the file to fs2.bat and launch it to play FS2. Create a shortcut for this .bat file on your desktop or on startmenu
W2k uses priorities for processes, by default every process is started as "normal". You can change it manually by using task manager but launching it via .bat file is easier. Mostly it is recommended to use "real time" protity, but for me it caused my mouse to lag badly.
More tips and tweaks look at this site: http://www.3dspotlight.com/guides-os.shtml
There is also a good services guide, definitely worth of looking.
Latest drivers for your Geforce or Voodoo or whatever you have, and if you are using a motherboard with VIA-chipset, download latest VIA-drivers and install them, http://viahardware.com/
Remember to install VIA drivers first, then drivers for your video card, and remove old ones first.
[This message has been edited by Mr. Fury (edited 05-29-2001).]