Is there a program that can pause another program?

Solution 1:

Use the Resource Monitor provided with Windows.

1. Open Task Manager
2. Go to Performance Tab.
3. Open the Resource Monitor in the left bottom .
4. Find out your process in the list.
5. Right click, there is option to "Suspend Process".
6. You can "Resume Process" afterwards.

No need to download any software, explore the Windows Operating System.

Solution 2:

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.