Thanks for the reply!
I got it to work, the dialog box for Virtual machine manager doesn't provide a way to create a new file, this is what threw me off.
I had to select the USB harddrive and in the next step, enter the name of the new file manually.
Once I did this it went and created the the file to store the guest os in.
I installed XP on it but when it restarted to complete the process it won't boot, something else I'll have to figure out
Edit: Got that sorted out as well