Just got a new computer with Windows 7 x64. Constantly was getting "Not Responding" on IE8 many different websites as well as some programs. Didn't see any correlation....
Found a mention and tried these:
Installed Windows 7 Upgrade Advisor from Microsoft. (even though Win 7 was already on the computer). AND in your IE8 click on the Compatibility Settings and check "Display all windows in Compatibility View"
I haven't had a problem with "not responding" since! Hope this works for you.