Garbage displayed on monitor screen?

I just newly bought from the computer shop an AGP video card and installed it on my computer.
Then turn on my computer and appeared on the monitor screen a garbage display.
I turned off my computer and detached the video card, then connect the monitor to the built in video card and turned on my computer.
And it was okay the display was good and running.
What’s causing garbage display, the video card is new?
