Had this one recently and was driving me crazy. Basically was looking to optimize logon performance in our xenapp environment and as part of it was slicing and dicing our existing v.large GPO into smaller more logical components. As a result of the slicing and dicing all looked good apart from this strange issue with IE.
If i went into IE and hit google i could type in the search bar and all was ok, only problem was when i went to other sites i couldnt type in the dialog boxes, yahoo,gmail you name it, went back to google and be the same.
This one took me a long time to uncover and in the end the culprit was activex filtering. Unticking
it from Tools | Safety | ActiveX filtering resolved the issue
To set via group policy