If multiple copies of W7 are running at the same time then you are supposed to have multiple licenses. You will probably have trouble activating the virtual copy if you use the same license key for host and guest OS, since the VM counts as a different PC.
Why are you trying to play videos inside a VM? The performance problems may not be CPU-related; the VM likely does not have full access to the GPU's hardware acceleration capabilities.
The years just pass like trains. I wave, but they don't slow down.
-- Steven Wilson