The No Add-ons mode allows IE 8 and IE 9 to temporarily run without any add-ons like toolbars, ActiveX controls, etc.
The No-addons mode in Internet Explorer is rather useful when you want to trouble shoot incompatible browser add-ons or extensions.
It is also very useful when you want to troubleshoot a problem like say freezing of your IE, to identify whether it is related to any add-on.
To run your IE in the no add-ons mode:
Start > All Programs > Accessories > System Tools > Internet Explorer (No Add-ons)
Alternatively, you can simply sun the command line arguement;
This will start IE with no add-ons.
Then through Internet Options > Manage Add-ons, you can enable/disable each add-on to identify which, if any, add-on is creating problems.
If you want IE to always start in the no-addon mode, you can also create desktop shortcut using the above command line argument.