I am seeing the same thing.  I am running Win 7 home Pro x64

I am seeing the same thing, I am running Win 7 Home Premium x64

I tried re-entering my serial number from help|about PowerDesk| change serial number, and after that the update completed without error.

I uninstalled the program, reinstalled from scratch and then the update worked.

I had to restart PowerDesk as an Administrator, changed the Serial Number (same as before), and the Update installed.

Several solutions have been submitted by others, but I want to submit my solution in detail and in as simple terms as possible, to help those who might be "computer challenged.

I was able to complete the update by trial and error by the following process:

First, open PowerDesk in Run as Administrator mode.  This is done by right clicking the PowerDesk icon and then selecting Run as Administrator.  When PowerDesk opens, click Help, About Powerdesk, Change Serial Number, then enter the Serial Number you were given and then exit PowerDesk.

  Again open PowerDesk in Run as Administrator mode and when prompted to update, follow instructions and the update hopefully will install without any problem.  Restart the computer.  After restarting the computer, PowerDesk should open without prompting for an update and Help, About PowerDesk should show PowerDesk updated to Version 8.4.1.12. 

The solution in my case was simply to change the Serial Number as Administrator, exit, and then run the update as Administrator.

To avoid any possible future problems, you may optionally change the PowerDesk Properties, Security, Privilege Level, to Run this program as an Administration.  You can change properties by right clicking the PowerDesk icon and clicking properties.

I hope this helps others who experienced the same difficulty I had in updating PowerDesk.