If you are using the vSphere Client to access vCenter or the ESX host directly, when you right click the VM and select Edit Settings, there should appear to be several tabs, one of which is Options. Under it is Guest Operating System select thereunder Windows 7.
It is possible that VMware Permissions were set that could prohibit you from "seeing" this option - but that is not the default behavior (If your VM hardware version is version 10 or higher you need to use the vSphere Web Client to accomplish your goal)