I'd have to second or third the trigger control issue. We had the 8 pound setup here, and had some problems with our weaker shooters. We changed to the standard 5 pound setup and now everybody is fine. Work on that trigger control and I'll bet the gun will group.

The first Glock I ever shot was a 23C. I did a quarter-sized group at 5 yards.

I own a G17 and will probably be getting a G26 soon (both 9mm).

I have found with the Glock what works best for me is to put the dot on the front sight in the dead-center middle of the square the rear sight forms. (Think of the rear sight as the lower half of a square.) Place the dot over where you want the bullet to hit.

Like others have said, the trigger pull on the Glock is critical. I found doing the $0.25 trigger job helped smooth the pull tremendously:

http://www.alpharubicon.com/mrpoyz/glock/

I went down to the 3.5 lb. spring, but my accuracy suffered and I went back to the 5 lb. spring.

Oh, and I've seen several "C" model Glocks at gun stores with warped front sights (though that wouldn't lead to your problem). It seems the heat coming out of the vents has a tendency to do this. Did you buy yours new or used?

My 23 is fine. Perhaps the "C" factor is kicking in. Trigger made need a little tweaking or maybe some aftermarket sights.

Mine works for me, close range is what they are made for. Have you been taught the proper way to shoot?
Self defence shooting is different than target shooting.

I bought the gun new about a month ago. and have only put 150 rounds through. Thanks for the link I'll have to give that a try when I get some free time.
thanks