XP SP2 RC2
You IT professionals out there (you know who you are) probably already know this (because, you are like the mythical town of Lake Woebegon, Minnesota, where the women are strong, the men are good looking and all of the children above average intelligence(r)), but if you haven't started testing Microsoft's latest Service Pack 2 Release Candidate 2 (instructions for installation here or here), get it now. There are changes that will break applications. Many of these applications are network related and will fail because of the firewall that SP2 enables by default.
The Release Notes list some of the applications that will have to be updated or modified. Read the other information on the MS site to find others.
Also, be on the lookout for updates from your own PC manufacturers.
The bottom line, in a corporate environment anyway, is that SP2 may cause some problems. How many and what kind will depend on what you are running. Most of you should be okay, but not everyone will be so fortunate.