Normally I would recommend Adobe Premiere, because that's what my friends have recommended to me. However, you can only get the CS5 free trial version (which only works for a month, and requires a 64 bit PC to use), unless you pirate it or spend hundreds of pounds on the software, and I wouldn't recommend pirating. However, I have had nothing but trouble with that, as I've been trying to get a decent editing software kit for a theatre trailer I've been asked to direct/shoot/edit, which I finally completed today on Windows Movie Maker. Because all week, CS5 has made my computer crash, to the point where I've gotten rid of everything Adobe-related on my laptop now except for Flash, Shockwave, and Reader, which are perfectly fine (touch wood).
Other good ones I've heard of, and that my fellow film-making friends use, are Avid and Final Cut, but I'm not sure if those are freeware or if my friends pirated them, or if they're fairly cheap, or what. I'd recommend you check those out and check the prices, then decide if they're right for you.
Really, anything is better than the standard stuff that come with a Mac or PC, though a Mac is definitely the better option to use when doing anything relating to film. In fact, for anything creative, it's better to go Mac, if you're doing office based work then PC is the way forward. The software, therefore, should reflect that. But you should be fine with the basic default stuff like Movie Maker if it's for an amateur project, if it's something more professional then you should check out Final Cut and Avid, and their prices, ASAP.
Hope this helps

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