There could be many reasons for this but probably the best place to start is the amount of video you are trying to put on 1 dvd.  if you are trying to get more than 4.7 GB of video on to a DVD by using a "decryptor" or some other form of compression,product  you may need to search for one that works better for what you are trying to do.  Second, if you are using large video files (and .avi files are pretty big)  you need to look at both cpu speed, and the amount of RAM you have on your system.  You also need to understand how many other applications are running on your system when you are rendering your video to make sure that enough CPU and RAM is available to properly render the project.  My system has 6 GB RAM and runs a AMD P940 Quad Core processor and for large projects that does not seem to be enough if there are other apps running in the background.  For these projects I usually shut down everthing else that is running including anti-virus (after disconnecting from my network) as well as most of the programs that automatically run at windows start-up,  clean out my swap space, and cache, then render the project.  I have never had a problem using this approach for large projects.  There are some other less common reasons for this, so if you need more information, Send another post and please include a little more detail about your system, cpu, memory, available disc space, project size,etc. 

One final thought, have you checked your hard dics fragmentation level?  A severely fragmented hard drive could contribute to this and many other problems.

Good luck