I have the same problem.
did you ever find a solution?
Originally i installed and ran wp on apache on vista business. worked fine including upload of images. Long story short, moved wp over to iis7. seemed to work until I tried to upload an image. same error message “Failed to write to disk.”
Not sure why this is not a support topic. My dev machined is vista. I need to upload images to test. IIS permissions seem fine, but who knows. Also I can find no trace of an error outside of the message displayed.
