I’ve run into an error at times when trying to install/update clients ShoreTel Personal Call Manager (PCM or the ShoreTel software). 

Looking through multiple forums I found that this is a known issue with ShoreTel.  They have traced it back to some files getting corrupted on Windows XP when SP3 is installed.   There are two Solutions to this problem.   Unfortunately they are not quick fixes…

First you can go through the steps shown in MS KB891801 – How to reinstall Microsoft Distributed Transaction Coordinator on a computer running Windows XP. 

Second you can uninstall XP SP3, install the ShoreTel PCM and then re-install XP SP3.

Unfortunately both solutions take multiple reboots.   If you are knowledgeable and feel like trying the KB article you I recommend taking screen shots of your current services before disabling them so you can get them reset back the way you had them (Before step 2 ok KB to disable all but the listed ones, and step 7 when you re-enable services).