cannot uninstall/upgrade Widcomm bt software, WinXP SP2

My Widcomm 4.3 program developed a fault & when I tried to uninstall it
I got an error message to do with a missing msi file or something. I
tried hacking all references to Widcomm software out of the registry
plus all of the relevant dll files but any attempts to reinstall it or
upgrade it to v5 generates error messages. I also tried reinstalling it
when logged in on an administrator's account but still no joy. Can
anyone help with any suggestions as to how I can make my computer
forget it ever saw this software?

It no longer shows up in the add/remove programs list but I suspect the
relevant folder I need to delete is a Windows Installer one.

Thanks