Honestly, that 1GB of RAM might be your problem. From what I understand, just Vista alone can suck up that much. 2GB of RAM is better, and I would seriously consider upgrading to 4GB or more.
And honestly, I would rather have bought a cheaper, older, more reliable version of Windows, and then bought a nicer editing program for your video work. I know one guy who still uses Widows 2000, and he's happy with it. His hardware is state-of-the-art, and the smaller software runs super-fast. Personally, I like XP just fine.
Vista is cool, I'll give you that, but I can almost guarantee that running Vista is the cause of your problems. Vista is a resource hog, and while your PC is about the minimum specs that Windows recommends for Vista, and the word on the street is that Microsoft's minimums are too minimal.