I have a computer with a Unix OS installed on it. I want to convert this computer to a virtual machine so that I can run it on another computer with VMware workstation.
Basically, the VMWare Converter will take the existing physical Unix machine and convert it into a VMWare format. This VMWare image can then be loaded into whichever VMWare host you choose.
Although I prefer the VMWare Converter answer, you could probably also do it using Clonezilla. Just be careful to create a VMWare image that has a virtual hard drive that is the same size or bigger than the original. Also, keep in mind that when the virtual machine starts up its going to detect totally different hardware and so you might have an issue with X-windows, network connectivity, etc. that you will need to work around.
If by UNIX you mean Linux, you can use VMware Standalone Converter.
But you must have a running VMware Hypervisor / ESXi system as the destination.
You cannot just create the files that are then opened in VMware Workstation or Player (or Server). [When converting a Windows system, you can just create the files directly.]
As for other "UNIX" I believe only Linux is supported and I know for sure (from trying it) that FreeBSD is NOT supported for conversion.
Note that the ESXi Hypervisor can either run on bare hardware or can run in a virtual machine - if you have newer hardware (that supports VT extensions in the CPU) and a new enough version of VMware Workstation (8 or better).
Also note that you control and manage ESXi / vSphere Hypervisor from another program - the VMware vSphere Client - running on Windows!
For instance, I installed ESXi 5.0 as a VM on VMware Workstation 8 hosted on Windows 7 on a 12-core AMD Opteron system. I ran both the Converter and the vSphere Client on the Windows 7 host system.
This was needed to convert a 64-bit CentOS 4 system to virtual. My older ESXi 3 system hardware would not support 64-bit guests.
Once the CentOS 4 system was converted to a VM under ESXi 5.0 under Workstation 8.0, I exported it to run directly under Workstation. I then shut down the ESXi VM.