Disabling it will have no beneficial effect on the temps and would cause performance issues within Windows. While I agree such a temp is high, it's within the GPU's thermal threshold.
If you're up for it, sometimes removing the HS/Fan assembly and cleaning the card and HS/Fan can show significant benefits. Of course you'll have to apply new thermal grease (TIM) to the GPU itself, and you may also have to replace torn/worn thermal pads should your card use them. Fortunately, neither the pads nor TIM are at all expensive. $4 for a 4"x4" sheet of thermal padding (must be cut to fit) and roughly $5-$10 for a syringe of quality TIM.
When I said I was getting a new PC "shortly", I had no idea HOW shortly. The UPS just dropped a yummy package of i7-930/Radeon 5850 on my front porch five minutes ago, about three days ahead of schedule.
Thanks for the reply, but I think have a solution. The next sound you hear will be a plug being pulled. I shall now play "Taps".