Originally Posted by Saracen
There are plenty of reasons not to switch, depending on your circumstances.
I'd go one step further and say that's practically no reason to make the switch. If you have a stable PC running WindowsXP, there's nothing to gain from making the switch to a new OS at the moment. There's precious little "Vista only" software out there and the gains for gaming have yet to materialize.
It's not that Vista is a bad choice for an OS, far from it, on a new machine it's a better choice and I personally am quite happy with my Vista experience, but I can't ever see a reason to upgrade from XP.