There is a viclient that is possible to install on 64 bit windows included in VirtualCenter 2.5 Update 1 (and newer), ESXi update2  and VMware Server 2.0.

This was a problem in older versions of ESX. For solutions if you don't want to upgrade see this thread in the VMware Communities: VI Client 2.5 only supports 32bit OS?" and an even better HOWTO written by Douglas Brown at http://www.dabcc.com/article.aspx?id=6826.