I just tried it with no luck--I unchecked mirroring in the mac side then switched to the PC side. When I switched the same thing as always happened; namely, the monitor registered no signal until I went into the display properties and checked 'extend my windows desktop onto this monitor.' Then when I tried to check "Use this device as the primary monitor" it grayed out the second monitor and took away the image from that screen.
I'm so confused, I have no idea what happened and why it won't play nicely anymore!
Thanks for your help!