With newer versions of ESX/VC (3.5u1/2.5u1 and newer),  installing under 64 bit windows is not a problem anymore.

It used to be 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.