Your motor is bad. A very common problem. It will cost less than $100 to do
it yourself. It's kind of a pain in the a.s. The trick is to wedge in a pair
of rubber door stops where the glass goes into the top of the door. this
will hold the window locked into position while you remove the regulator and
install the new motor. I would guess about two hours labor for Chevrolet to
do it for you. - Gary

Step by step picks that I've used on two installs.  Took me approx 1 hour
the first time, 45 minutes the second time.  I bought both motors for under
$50...PepBoys I do believe:

http://csce.uark.edu/~jgbertr/rb/alt_web_page/windowmotor.html

If you are even slightly mechanically inclined, I'd recommend saving your $$
and DIY.  Call Chevy and ask how much...I bet it's at least $150.  I didn't
need to use the rubber door stops on either car.  I used a metal bar wrapped
in a towel.  There's a reinforcement bar in the door that you can rest the
bar on, and it will hold the window up if it decides to drop while the motor
is out.  My windows, however, never moved while the motor was out.

Travis