visual studio installer problem

Hi,

I am using visual studio installer for installation of our unified instant messenger.We have released about 40 versions of our UIM till now and everytime there are dll's with new file versions.According to an article in MSDN (The first article if you search MSDN with "product information tab"),for every release I used a new product code(GUID) and kept the upgrade code same.The upgrade code being same is supposed to uninstall the older version and install the new version.But this is not happening instead I found 40 instances of our product in the client machine.(I checked it through add/remove programs)If i keep the product code same,there is no way that the new dll's are overwritting the old dll's.
If I try to uninstall manually,sometimes the system is crashing also resulting in some lost data(as complained by the clients)I also got a complaint that other microsoft products like outlook are not working properly because of this visual installer.Do visual installer have dependencies on other Microsoft products.

Regards
Praveen