Yes re windows key to unlock the mouse,
Yes re giving up the mouse whilst the remote is being used, you can use both but one will steal focus from the other, windows will only accept 1 command at a time, same as you can't type and move the mouse at the same time,
The remote is just another input device,
Ideally a dedicated media centre pc is best rather than trying to do too many things at once on a single pc,
Finally, the MSI 8600 is adequate.
If you have a dedicated MCE pc, used for nothing else or very little else (no games) then you don't need a powerful machine and the modern inboards graphics is adequate without need for a separate video card.
Hope this helps.
Someone else "interacting" with the TV I gather you mean using media centre at the same time as you - you can only use a single input at a time, do someone changing a media centre tv channel will jump you mouse to the media centre screen, you'll have to move your mouse back to whatever else you were doing beforehand. In short, you have 1 pc regardless of how many monitors there are, the pc will multitask but you will only take a single input at a time.