It was not until WinXP that they merged the two OS lines together, and apparently successfully so in the workstation market although I still struggle to get games running on this, something I do not have trouble with in Win98SE.
Win98 is neither a DOS application or not designed for games. Indeed, 98SE is probably the best OS that Microsoft has released. This misinformation that people spread about Microsoft operating systems is quite amusing, but doesn't help others resolve their issues.
The vendor did you no favours in convincing you to go Win2K. Chances are he/she did that ONLY because it was the cheaper option so they could make more money. Depending on the age of the machine i'd be inclinded to take it back, along with the game and the modem (if they sold you that as well) and make them fix it. After all, they recommended the OS, and probably installed it as well?
People who work in shops generally have very little understanding of the operating systems and how to install them correctly, they usually just follow the bouncing ball and give it to you.
Now you have it you may need to get out on the net, upgrade Win2K to the latest service pack and hotfix (critical if you are on the net as Win2K is quite insecure in it's original format) and update the drivers for all your hardware (modem, video etc) to the latest versions. Look at updating the firmware on cards/modem etc as well and you may find that things will work better after that.
Additionally you could try formatting your hard disk drive and re-installing Win2K and then do all the upgrades
if all else fails put your PC behind your car and run it over .. won't fix your PC but you'll feel much better