This is was a weird one.. whenever I tried installing the .msi file to install Microsoft Office Communicator 2005 on a Windows XP SP3 machine the installation would fail saying it has been interrupted which is really annoying since this was early in the morning and I was still waking up!
Anyway after looking around I found couple of fixes:
One was that some people were able to fix this by un-installing the Security Update KB974571 from the XP machine but as for me I could not do it because when I clicked on this update in the control panel it said "This update cannot be un-installed" and it was from way back in 2010 so I thought it might not be my solution!
So over to number 2 which is to download this PATCH and run it on the machine, it cleans up the registry for any tombstone entries that might be present for MOC 2005 and takes about 5 seconds to complete. After that when you try and run the installation it will finish without any issues!
Take that you ... Office Communicator!