Only use Windows XP (currently as part of VirtualPC) to run apps for which no Mac version exists of a Mac version lacks functionality. The list includes UCINET/Netdraw (used for analysing social networks), SPSS (stats package - Mac version is always 2 versions behind Win which causes no end of problems sharing files between platforms), and AMOS (for structural equation modeling). As the BBC has decided to launch its TV catch-up service on Win only will also have to start using Int Exp and Win Media Player again - the worst of all possible worlds!
I use windows (via parallels) mainly for visio and handling Hebrew office documents. Another thing is using Imagemagick's convert utility (I didn't want to upgrade the one installed as a part of the MacTeX package before I'm done writing). The rest (net-related time-wasting, numerics, writing) I do on the mac.
I was going to say for looking through at the garden and keeping out the rain but it seems we've been through all that already!
I'm happy to say I haven't had to touch Windows since changing to Mac about two and a half years ago. There was a demo copy of Microsoft Office on the PB when it was new but it was too aggressive in jumping in first to open everything and I got rid of it after a few days.