I've been using Adobe Photoshop Elements for about 6 years now. It has most of the power of it's big brother, but simplified for non-Pro users. It's also alot cheaper, only $99-. If you get the retail package, it comes with both the Mac and PC versions. Just make sure to get the latest 9.0, I still see older versions around in retail.
Elements also has a much smaller hardware requirement, Dual-core laptops with only 3gigs of RAM can easily handle it. On my i7 quad-core laptop, I've yet to have to wait for anything, everything happens in a blink.