Windows 7: I have recently upgraded from IE 9 to IE 10. The problem(s) occur when I try to interact with sites; for example I want to place an up vote here in SuperUser.
Another problem was I could not post in Facebook.
Workarounds: a) Use Mozilla. b) Use another machine with IE10!
Suspicion: It's something to do with my Google Toolbar. That said it was in the same state as when I used IE 9.
N.B. This is a screenshot of the problem, and not an answer!

2 Answers
Disable all IE add-ons (including Google Toolbar) and check if the problem persists. If it's fixed, try enabling add-ons one by one and check which one causes the problem.
Also try removing cookies. If you have 3rd-party cookies disabled, enable them.
1I have found recently that simply clicking on the X and continuing solves the problem. The dialog box, or 'Up Vote' then accepts my command when I click on it a second time.