If you want to convert from DVD to any other format, go for InterVideo DVD Copy.
If you want to convert AVI files to MPEG1 or MPEG2, the best software available is CCE SP (CCE : Cinema Craft Encoder). CCE SP is a shareware and if you want to buy it, you will have to shed about 1 Lakh from your pocket...But no doubt, it is the BEST encoder available.
For more information on stuff related to Video, go to
www.doom9.org or
www.afterdawn.com