You shouldn't need to buy anything.
While I'm no great fan of Microsoft, their own (free) video editing software is actually quite good. If you're using ME, XP or Vista, you should already have Windows Movie Maker on your PC, with some advice on using it being available here on the Microsoft website:
http://windows.micros...h-Windows-Movie-Maker
Windows 7 users need to download Windows Live Movie Maker 2011:
http://explore.live.c...dows-live-movie-maker
However, if you'd prefer something truly professional, the software used to make many major TV series and feature films (e.g. 'The King's Speech') has been placed into the public domain and it can now be downloaded totally free of charge:
http://lightworksbeta.com/
Chris