XP and applications

Is Windows XP so different from 2000, NT, 9x that other software vendors will have to make changes to their applications so that they can run on XP?

I’m not worried about any specific applications, I’m just curious if something like this causes a big headache for software vendors who’s apps run on Windows.

http://boards.straightdope.com/sdmb/showthread.php?threadid=91840

ok, smack me!

Thanks for the link.