You don't "uninstall" a virtual machine. From within the virtual machine software, you can have any number of virtual machines configured.
So, let's say you create a Windows 7 virtual machine with 1GB of RAM and a 40GB hard drive file. Depending upon the virtual machine software you use, the file may start out small and grow over time to 40GB..or it may start at 40GB right at the start. So, you install the OS and you use the VM for awhile, but then decide you no longer need/want it. So, you go into your VM software, find the virtual machine, right click on it and choose Delete. It will ask something like (do you also want to delete the associated virtual hard drive files from the hard drive). If you say yes, it's gone.
So, you can get the space back when you delete the virtual machine, or you can optionally keep the virtual machine hard drive file to use as a starting point in the future.