Is seems a little bit strange that the virtualization tool disables virtualization.
To understand this, you need to know what is going on with your system when virtualization is being used.
When you don't have virtualization enabled, your system acts like this:
[ Hardware ] -> [ Operating System ] -> [ Applications ] -> [ You ]
When you do with Hyper-V, there's an extra layer in there:
[ Hardware ] -> [ Virtualization Hypervisor ] -> [ One of possibly several OSes ] -> [ Applications under that OS ] -> [ You ]
By enabling Hyper-V, you have a hypervisor in there, even though you don't notice any difference. Now, notice that the hypervisor is directly above the hardware with Hyper-V. VMWare server products act like this too.
So, when Hyper-V is enabled, you are "in" a hypervisor, even if you are only running 1 "guest" OS (your pre-Hyper-V OS) at the moment.
x86 hardware does NOT support "nesting" hypervisors. The VMXON instruction that sets it up will cause a "VM exit" if hardware virtualization is enabled. Thus, when a hypervisor is active, if the OS in that hypervisor tries to set up virtualization, typically the hypervisor will just tell the guest OS that virtualization is not supported.
It's possible for the hypervisor to "emulate" hardware virtualization if it really wanted to allow guest OS's to use virtualization features, but as you can see, Hyper-V has chosen not to do this (it's complicated to do, anyway).
Now, if you are using VMWare Player or Virtual PC in such fashion:
[ Hardware ] -> [ Operating System ] -> [ VM Application running an OS in it ] -> [ You ]
VMWare player is able to keep multiple instances of itself straight (it installs a number of drivers), so you can run multiple VMWare players without issue. I'm not sure if you tried to run VMWare player and Virtual PC though - it probably wouldn't work.