+1 for CamStudio, although you may want to change your video & audio settings to compress the video & audio more than the default codecs do otherwise you will end up with a multi Gigabyte size movie before you know it and could eat the crap out of your cpu/memory so you end up with a video that freezes during your tutorial.
As for audio its all about user preference and how crisp you want the sound to be, make sure you have your mic plugged in before you try to adjust your mic audio settings, you can also refer to the
camstudio forum for more tips and answers to any questions you may have concerning Camstudio.
Then after your done recording you can Edit your video with a
Free Video Editor or you can use Windows Movie Maker/Windows Live Movie Maker which also does a good job at editing videos, this way you have an intro and or other effects in your video tutorials so they look more professional and give the veiwer a greater tutorial experience.
try these video settings in camtasia or just adjust them until you find your own preference. You may also have to install codecs like xvid if you dont already have them, K-Lite Codec Pack is a good option as far as codecs are concerned, I use the K-Lite Mega Codec Pack wich includes just about every popular codec from
CodecGuide.