For one, you may just have a slow computer. Maybe you're scene has too many shadow lights, are you using fog? Glow? Displacements? High polygon models? These are things that will slow down you renders.

As for the AVI, I never render to AVI so I'm not sure what to say. I always render frames (SGI, TIF, TGA, even JPG) and then compile a movie file later. That way I can see each frame as it renders. Often my renders can take 5-30 minutes a frame. 500 frames of that would take a long time. If your computer starts to choke after a while, you could end up screwed.

Also, I didn't know you were doing this in Maya. By rendering within Maya, a lot of your Ram is being used just to keep the application open. I recommend closing Maya when you're ready to render and render through the command line interface. I wrote a simple dos batch file once that I'll give you. It's really simple.

Edit my file in any text editor (notepad), and change the project field to where your project lives, the images field to where you want the renders to go, and the file field should point to your Maya file. Leave my render global overrides alone until you get more experienced. Save the file. Change the "txt" extention to "bat", and double click it.

I hope this helps.