Omeka doesn't support Windows, and while there are no plans to support it directly on Windows, people seem to have the best experience using a virtual machine running some flavor of linux. Here's a recently-updated forum post on the topic:
http://omeka.org/forums/topic/omeka-ubuntu-vmware-win-or-mac
Kris
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http://www.9while9.com/index.php?option=com_content&view=category&id=23&Itemid=30
Under "Omeka 1.3.1 Virtual Appliances" at that link, there are
preconfigured virtual machines of Omeka 1.3.1 for both VirtualBox and
VMWare.
-John