5 mins is about 1Gb for DV format video. You need to allow for:
- The original footage that you capture
- Some working space for transitions, stills and music
- Output of the edited video
Then there will also be a converted file if you want to output to DVD or Video CD etc, but this will be much smaller - about 2 hours on a 5Gb DVD.
So you should be OK for a 30 minute video assuming 60 mins original footage (12Gb), 30 mins final version (6Gb), DVD file (3Gb) and 2Gb for working space, music etc - total 23Gb.
Normal advice is to use a separate drive for your video footage as there may be performance problems due to contention if you use the system drive - but this will vary depending on how fast the PC is.
Filming problems to look out for are focusing on, or reflections from, the window and camera shake.