Time Lapse photography- my new goal!?

[noh moh muffahns]

Hello everyone, I'd really like to get good advice on time lapse photography, if you can answer a small portion of my question- great. If you can answer it all- even better. But let me get started!

First things first! I have a Cannon Powershot S3 IS- absolutely great camera if you ask me… And I noticed it had the intervolmeter built within, then I set it up and deciding to try taking a small time lapse photo of some clouds- well, this didn’t turn out too well… I had it set at the lowest interval – which was every 60 seconds it would take a photo- well this is way too long I quickly found out. So I’m guessing if I want to pursue this I’m going to have to click the trigger every 15-25 seconds if depending on what I’m taking my time lapse of. For a sky time lapse image though what is recommended interval and then playback speed?
Also I know the basic concept behind time lapse photography but what is a good (preferably free) program to stich it all together? I have Pinnicale Studio 9 just haven’t used it much (it’s our families not mine lol)- and the basic Windows Movie Maker which I was using…

Also I’ve heard using your webcam is a good way to go for getting the correct time in-between shots- but the program from Microsoft only works with Windows XP I have Vista- which free program is there that I could use?

Ok this might seem a bit jumbled etc hope it all made sense, thanks to whoever can help me out.