-DivX installs Google toolbar silently, thereof is considered spyware by a lot of people an not installed (eg. me).
I installed DivX this morning and was given the choice, unfortunately I was expecting a Finished button and clicked Next too soon... The Google bit does have a seperate uninstaller though and so far, touch wood, no problems.
I certainly didn't realise Google where using my download bandwidth though, I wanted DivX, if I want Google software i'll go to google.
That's all bye-the-bye though as the main question here is codecs. I think .mpg is the safest solution for decent compression/quality and support. None of the standard codecs with Windows give very good quality and the filesizes are massive.
I've used vp31 a bit recently, which is free and has a small codec installer, but of course I have to offer the codec install whenever I post a movie file.