Dock Problems with Leopard - no icons

I did a search and could not find this problem so please forgive me if this has already been answered. I am using Leopard on an aluminum imac. When I launch applications from the finder window the icon for the running application does not show up on the dock. When I quit applications that originally had an icon in the dock the little blue light remains on even when I do a force quit
I did a fresh install and the issue persists
I am not an expert so it could be a simple thing that I am doing wrong

Hi-
I was able to fix it. I noticed the DIvX thread above and while it didn't describe my dock issues it did make me realize I had recently installed Divx. I deleted it and restared and now everything seems to be working.

Hate to revive a dead thread that I started but this problem persists. I thought I had fixed it a while back as my previous reply states but ... I continue to have difficulty with the dock. If I launch an application from the finder the icon does not appear in the dock. If I launch an application from the dock more often the not the blue dot wont appear under the icon (although the program will load). When I try quiting it appears to have quit but when you check the force quit I find that all the applications I thought I quit are in fact still running. What gives ??? I am not the most proficient user but I do highly prefer OS X over windows but this has really shaken my faith in Apple