T.G.
I've heard VideoSpin is real good. For simple stuff, like what it sounds you want it for, I use Picasa (a free photo editing software, and movie maker). http://picasa.google.com/ It's very user friendly. Yes, it's primarily good for photo enhance, but it has a movie making (using photos) option as well. There is also a Video Editor on YouTube http://www.youtube.com/editor (more for after you've created a video, decent for transitions and adding music if you haven't already). Both YouTube & Picasa are Google, so they all work real well together. Plus, these are all super simple to use. No large learning curve. As someone w/ experience using full editing software I can tell you they're much easier to use. You can't expect to be able to do everything you can do w/ bought software. But you can do LOTS!
If you're gonna download a trail, you might as well use Adobe Premiere of Final Cut Pro. Or, whichever software you're experienced in.
Good luck! Let us know what you end up using.
You'll like Windows 7 much better. But I was advised when I was building my new system to NOT put Vista on, then upgrade. So, do your research. It may be better if you have a "clean slate" before installing. Oh, and if you don't already one, you'll want to upgrade you system to a 64 bit system before installing.