For some companies that's not viable
I work for a major company. We have tens of thousands of desktops all running Windows 2000 currently. Many moons ago, our company made the decision to skip XP. As many people know XP was less than exciting when it was released. As a matter of fact it contained many problems that my company was not willing to deal with. Since MS was supposedly on a 3-5 year release cycle, a decision was made to skip XP and wait for the next OS. Also, since the company had already migrated to 2000, it did not make sense to quickly migrate to XP (which was released only a year later).
So now we have a problem. Microsoft is going to end support for 2000. We have only 3 real choices here. #1 upgrade to XP. This doesn't make much sense as XP is almost as old as Windows 2000 is. #2 upgrade to Vista. This is the current objective and they are very close to production rollout (despite the problems we have had during testing). #3 stay on 2000 longer with either A paying extra cash for support from Microsoft (security patches mostly). Part of our business is going to do this as their part of the business is far to valuable to risk major downtime with Vista and performance issues, or B no support, which is not an option, but I there it in anyway.
Going to Windows 7 isn't an option at this time. There isn't near enough time for a company this size to test Windows 7 against all the software used in this organization. Vista has been tested. There are some successes and failures, but it's been tested. No OS has a perfect test record. 2000 didn't either when they moved to that either.
As for "virtualization" someone else mentioned. This is not an option either. Although some desktops who use light applications could possibly do this, it is not sufficient for our needs. I heard a story from an employee who has been with the company a long time that our company tried to go the thin client route many years ago. It failed miserably as a lot of applications did not perform as required, and the number of clients per server is miserable. We still sue thin client for some things, and it's an awful experience on the Windows side (the *nix side is actually fairly pleasant).
What do I do for the company? I am a Windows admin turned *nix admin. I was sick of fixing Windows problems 90% of the time and only 10% of my time spent making things actually better. As a *nix admin I spend over 90% of my time making things better.