Some people have had issues with security software getting in the way (adblock or something in Viper I think it was?).

Some others have found suspicious programs in the Add/Remove Programs list (appwiz.cpl), or running in the background when they pull up task manager that were the root of their issues (can launch Task Manager from the menu you get when you right-click the task bar, or hit CTRL-ALT-DEL). May be worth running a malware scan just in case.