For my G2, I use WinMovieMaker and re-render in AVI. Those new rendered AVIs will open in Vegas.
Otherwise, for (I guess) around $30 you can buy some codec on the web (Mainconcept ? don't remember)
Try a search with "powershot" and "avi" as keywords, the subject has been discussed a few times since, well, the Canon G2... ; )
Yes, the MainConcept MJPEG codec works well. The free download trial works for decoding (ie. reading on the timeline) but you have to pay if you want to render MJPEG w/o a watermark. I use it with my Canon S50.