She passes on the Mac because it has only 2 GB of RAM, while the HP has 4 GB of DDR2 (slower) RAM. She gets a screen that maxes out with a video resolution of 1366x768, while the MacBook Pro has 1440x900. Worse yet, she will get Windows Movie Maker instead of iMovie, and she can never run Final Cut Pro or Express, which she may want if she goes professional or semi-professional.
Of course, the Microsoft ads are pushing price, not features and quality, and there is some talk that Apple, seeing the recession take hold, is increasingly interested in some lower priced -- but hopefully not lower quality -- products to sell.
While there are certainly millions of buyers for whom a Windows PC would be a reasonable choice of computer, it's worth noting the long-standing mild irony that Crispin Porter & Bogusky, the ad agency that created this campaign for Microsoft, is (like the majority of advertising agencies) pretty heavily invested in Macs. Chances are, for this ad, the video editing that Sheila aspires to do was completed on a Mac. If you have an idle moment, check out this profile of the agency on the web and click on the office shots. I especially like the shot of Alex Bogusky's desk with two MacBook Pros sitting there. Different strokes for different folks.