For my own reasons, I have never updated my XP installation past SP2. Touch wood, this has given me few problems. The only problems encountered to date are a) the latest Skype update requires SP3, and won't run without it, and b) Google have changed something in the way Chrome security works, which means, on random perfectly legitimate (e.g. yougov, twitter) web sites, I get the following error:
Cannot connect to the real xyz.com Something is currently interfering with your secure connection to xyz.com. Try to reload this page in a few minutes or after switching to a new network. If you have recently connected to a new Wi-Fi network, finish logging in before reloading. If you were to visit xyz.com right now, you might share private information with an attacker. To protect your privacy, Chrome will not load the page until it can establish a secure connection to the real xyz.com.
It's a widely reported problem (and not, it seems, just for XP SP2 users).
IE 6 (!!) also has similar issues.
I've solved that problem by installing Firefox as a third browser... :-)
And yes, before anyone mentions it, I'm well aware of the security risks I'm running.