Because that's how long a Norton Internet Security liscence lasts. Renewing the liscense just seems like a waste of money when there are potentially better freeware products on the market. I use Avast! anyway because it's less resource heavy and intrusive.
Oh, and my XP install always seems to gain a million random apps that I use once and never uninstall, so it's nice to have a nice clean install every January! However, I have had to reinstall on various occasions where something has gone tits-up and I'm too lazy/clueless to figure out what it is. I like reinstalling Windows; you tend to find things you'd forgotten about in that mad rush to save what you want to keep before it's all washed away. Kind of like re-arranging your bedroom, or cleaning out your car..