Does Kindle4PC support 64-bit operating systems? It probably does, but I couldn't find anything about this in the information on their download page.
This article explains a little bit more about Windows DRM system -
http://support.microsoft.com/kb/891664 .
On the other hand, vermontcathy, this is a work computer. You might check with your IT department; perhaps they have set up some new security/work rules and have blocked the use of such applications as Kindle For PC.

Usually though, you wouldn't even be able to download the application, let alone install it, but I would still check with your IT department to see if they have any ideas for a solution to your problem
Holly
Edit - Whoops, DiapDealer beat me to it. He posted his comment while I was busy writing mine. What he said!!