Today was the first time I had tried to launch the vSphere client on Windows 7, which failed of course, stating there was an “Error parsing the ESX server”

A google search turned up the following blog:

http://www.virtualni-svet.com/2009/06/how-to-run-vsphere-client-in-windows-7.html

This worked for me, the only “gotcha” was the location to which the system.dll file is saved in Step 2, needs to match the DEVPATH variable specified in Step 3.