I did a stupid thing and I added an incorrect URL to my list of plugin repositories in NetBeans, and now Plugins hangs my NetBeans whenever I try to open it. I suspect that if I were to remove this bad entry from the list all would be happy again. But I cannot use the GUI editor to remove the entry because it hangs. Which XML file specifies the repository list?
Using NetBeans 7.1 on Windows 7, the location for update center entries is
%userprofile%\.netbeans\7.1\config\Preferences\org\netbeans\modules\autoupdate{Name_Of_Provider}.properties
where {Name_Of_Provider} is the name that you gave the entry. Deleting this file will remove the entry from NetBeans. You will need to shutdown NetBeans before you delete this file or at least restart it to pick up the changes.