Welcome to the VMware community.
Windows 7 supported on ESX 3.5 and Later hypervisor. We need to verify if that guest OS is supported or not.
http://www.vmware.com/resources/compatibility/search.php?deviceCategory=software
So, You need to have latest hypervisor in order to perform windows 7 or 8 installations. vSphere Client 2.5 is oldest version. The latest is 5.1 
I would advise you to upgrade the hypervisor verison.