Usually it's the browser or some addin being too busy to respond to Windows (the OS) asking if it's still fine. I don't consider this to be a bug but I think many will.
I am running Vista Home Premium with 2GB of RAM and plenty of storage space. The system is a two year old Toshiba laptop with a AMD Turion 1.9Ghz processor. For the last several months, I have been getting a lot of "Not Responding" messages, particularly with internet-based apps (browser, Thunderbird, etc.). It always eventually frees up and completes the task but it's very annoying. If anyone can provide any clues as to what might be causing this it would be greatly appreciated.