Does Xen/Citrix have a workstation/desktop tool to run virtual machines on top of an existing OS?
My host machine is Windows 7 64-bit Ultimate.
Nope, Citrix does not have a product for that scenario. They have however a Type 1 Client Hypervisor, the XenClient.
What Type 1 means is it's a baremetal hypervisor for clients, xenserver for clients if you want:) And on top of that you can install Windows or Linux OS's
VMWare Workstation is a type 2 Hypervisor, which means you gotta have an OS which is installed on the machine and the commands go through that OS, instead of directly to the hardware, a layer more!
As an alternative try Oracle's VirtualBox, which is a free Type 2 Hypervisor!