Isn't there a Starcraft 2 demo?
I think there's like a free to play version...
Here we go.
"StarCraft II: Starter Edition allows anyone to play the following content for free (all you need is a Battle.net account and Internet connection):
The first four missions of the StarCraft II: Wings of Liberty single-player campaign, including Mar Sara 1-3 and the choice to embark on Tychus’s first mission or Dr. Hansen’s first mission.
The first two Challenges: Tactical Command and Covert Ops.
Access to the terran race in Custom Games and Single-Player vs. AI.
Access to the following custom maps (map selection may rotate over time):
Xel’Naga Caverns
Shattered Temple
Discord IV
High Orbit"
Although using it to test a new PC is kind of pointless since it's minimum reqs are pretty low I think.