"Enable Third Party Browser Extensions" grayed out in IE
If none of your third party browser extensions (toolbars, etc) work open Internet Explorer > Tools > Internet Options > Advanced and check "Enable Third Party Browser Extensions".
If this option is grayed out it probably means this setting is managed by group policy. To enable it permanently you obviously need to change group policy
Computer Configuration\Administrative Templates\Windows Components\Internet Explorer\Internet Control Panel\Advanced Page\Allow third-party browser extensions
However if you only want to enable this temporally on a single computer or don't have ability to change GP settings you can do it via a simple registry change (must to have local admin rights to perform this):
Start > Run > regedit
Navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Internet Explorer\Main > Enable Browser Extensions change to yes
If you need to do more often, create a text file with with following text, rename it to extensions.reg and save on your desktop.
Windows Registry Editor Version 5.00
"Enable Browser Extensions"="yes"
Whenever you need to enable extensions simply run the extenstions.reg file.
Whichever registry method you use after a restart settings may get changed back by the group policy.