Win 7 (and Vista) are at least 11% slower than XP, when optimized, which in turn is slower than Linux.

The problem with the latest driver has already been reported here, and a similar problem was reported several months ago for CC1.2 cards.

What application are you using to determine the GPU Frequency?

If the GPU Freq. drops from 800MHz to 405MHz it's going to reduce performance.

You might want to try overclocking software or to rollback to an older driver.

What is your GPU, GTX 560 Ti or GTX 570 ?

270.61 has recently caused my GTX470 to downclock as well :((
Tried a rollback and its still at 50MHz - Off to do a full uninstall, and reinstall.

What's worse is that this driver is clearly damaging the video card; leave it running GPUGrid or another GPU task and you start to get features on the screen.

Fought through the lengthy process to uninstall all the NVidia drivers and I'm now trying 26724, again; it worked before, but so far only errors for GPUGrid tasks. Also tried MW and got 2 errors; as soon as I used the system they failed.

What gets me is that the 270.61 driver worked for days, and is still working on another GTX470 on Win2003 without the problems.

Different cards overclock differently. Overclock-ability can greatly depend on the manufactures design - one dual fan card is more likely to overclock well compared to 3 GPU's with single exhaust fans in the same case. On the other hand you could not have 3 overclocked dual-fan cards in a closed case without about 8 exhaust fans.

Different calculations tax the GPU in different ways, so power consumption and heat output varies from task to task.

You would be about 50 times less likely to see any errors running MW because the run time is so short, and as you say, the tasks are not viable from your MW account.

So what you need to find is a set of frequencies that work for all the apps/tasks you run; failures typically undermine overclocking (offset any gain from freq. increase). For GPUGrid there is little to nothing to be gained from overclocking the memory and a slight underclock can help stability. I suggest you limit yourself to 10% OC; it's often the case that a higher OC results in poorer performance, so you must check your task run times against similar tasks.

Always remember the more you overclock the more you stress your card, PSU and wallet. After a while the performance gain is not worth the increase in power (cost/noise).