Error adding Products to an Opportunity

Hi,

I have just installed a new instance of CRM 4

All aspects of the system work fine apart from when trying to add Products
to an Opportunity. When clicking on products from within an opportunity
nothing actually happens - i.e. no window opens. In the bottom left hand
courner of the opportunity it says error on page. All popups are turned off.
This issue occurs on teh server in a browser, on a client in a browser and
in outlook on a clients machine.

Thanks
James