It's really a coin toss. Depending on what graphics card/drivers you currently have installed and what you want them to install, they may or may not remove the old drivers. They likely won't tell you either. If you ask, they'll likely say they did (regardless of whether they actually did or not). Just a guess here, but they probably think if you know enough to check them on that, you wouldn't have brought the computer to them in the first place.
-Wolf sends