Issue with Outlook COM plugin

We have an Outlook COM addin that tries to add a custom button in the toolbar while sending a mail, on mail composition window (Inspector Window). This is expected to work with Outlook 2003 as well as Outlook 2007. When we compiled and installed it, it seems to be working fine with Outlook 2007. However, it is giving problems with Outlook 2003. - On some of the machines the DLL did not get registered. It gave an error. It got registered only after we copied MSVCR80.DLL. - On some machines, after it got registered like above, it did work fine. - On some machines, after it got registered, it showed the button only when the Email Format was selected? by user is NOT "MSWord as default email editor and viewer". When these two flags were turned off, the button did show up. We are looking for someone to fix these problems so that it works seamlessly in all conditions in Outlook 2003 and Outlook 2007. This fix could be a change in the code or some other setting on the user machine that can be done through our installer when we install the addin.