Here is whats bothering me. For example, I play two games, Assassins Creed and MW3. When my friends call me to play MW3 I minimise Assassins Creed to go and play MW3. But Assassins Creed still consumes valuable processors time (over 50%, memory is not the problem). I understand that process must be in the memory but can I pause it somehow to not consume processor?
Use the Resource Monitor provided with Windows.
No need to download any software, explore the Windows Operating System.
This can be done with Process Explorer, as noted in a comment above. Download and run it, find your game in the list of processes, right-click on it and select Suspend.
To resume the process, right-click on it again and select Resume.
I gave it a quick test, and it seems to work. Started the Torchlight II demo, loaded a game, then suspended the process. Went to play some DiRT Showdown, then came back, resumed the Torchlight II process and could continue playing the game with no problems. Also, while suspended, Torchlight II consumed no CPU or GPU time (according to Task Manager and GPU-Z).
So it's definitely possible with at least some games, though I can't say about Assassin's Creed. Only way to find out for sure is to try it.