Are you using the final release version?

I think the earlier versions (pre-release) had an issue with

Try associating .png images with 'Windows Picture and Fax viewer' or remove the file type altogether and then add back in again to restore its defaults.

See this for the pre release IE8 problem:

(Sorry, can't post links, under 10 posts!)

but google: FeedbackID=389191

Good Luck
Dan