Microsoft took stuff OUT of Windows which means that some software is no longer compatible, but you may not find out until you try and run it.

When my XP laptop died, I needed a replacement urgently, but could only buy one with Vista. I found that one of my ESSENTIAL programs, VB5, will not run in Vista. There is no upgrade path, because the 'replacement', VB.Net is virtually a new language. A rewrite in VB.NET of a large program is out of the question, several months of hard (and unnecessary) work, even if it is possible at all.

Luckily, my XP machine has now been repaired and I am back in business. Warning - DON'T buy a new machine, especially if VISTA is installed.