The easiest way (not necessarily the highest quality way) is to drop the AVI into Nero Vision Express. Nero is not free however. A higher quality but more time consuming and complicated method of doing it is to use VirtualDub[Mod] (free) to demux the AVI to an AVI video stream and a WAV audio stream; TMPGenc (free) to re-encode the AVI to a DVD compliant MPEG-2 video stream and (optionally) BeSweet (free) to turn the audio into AC3 if you want to save space over the WAV/LPCM (some AVI's already have an AC3 stream). Then you can use a DVD authoring program such as Muxman (free) or DVD Author [GUI] (free) to make the DVD, i.e. the VOB's and IFO's contained in a VIDEO_TS folder. Then you burn with your choice of DVD burning software, which can also be done for free; for example, use DVD-Shrink (free) to make an ISO of your DVD files and DVD Decrypter (free) to burn the ISO (that is my preferred method of burning all DVD files, even though I have Nero).