Recently I was playing with vs2005 and outlook 2003 add-in. It was working fine. But suddenly the add-ins worked no more. When i manualy add the add-in to the outlook 2003 I received the error message "<path> is not a valid office add-in". I searched the internet but found notting. Then I reinstalled office 2003 and visual studio 2005, the problem was still there.
I posted this question at
forums.microsoft.com and within a day I got the answer to solve the problem and what cause the problem.
The way to solve the problem is:
"
It might have been "hard disabled". Check the "Help"->"About.."->"Disabled Items.." list. 
And what cause the problem is:
The Blacklist system can happen if you stop code in the startup method and if this is then debugged and stopped the process at this point outlook/office will think the application has caused an issue in this point and blacklist. If you are using VSTO 2 this will black list all VSTO 2 Outlook Addins. albeit to be enhanced and be Managed Code aware in VSTO 3 and Office 12
the link to the post on the forum.