In my opinion, there are four main contenders in the image-editing software field. They are:

Adobe Photoshop CS3. Industry standard, more tools and capability than any one person is likely to use in a life time. Good help files, reasonably intuitive for most basic operations, but expensive at $700 and up depending on which package you get.

PaintShop Pro X2: About 80% of the functionality of CS3, inluding built in HDR merge functionality, and levels and curves. Uses most Photoshop plug-ins. Will do everything 99.9% of amateur and semi-pro users ever need. Must more wallet-friendly at around $90-110.

Adobe Photoshop Elements 6: The "light" version of CS3, contains most of the important functions. IMHO, not as capable as PaintShop Pro, but very close. Price typically around $110.

GIMP (Download at http://www.gimp.org). Capability between that of CS3 and PSP X2, but as this program was originally developed for the Linux OS, the user interface will be a bit of a departure for Windows users; not quite as intuitative, but once you learn how to use it VERY powerful. Price? Free!

If you have $100ish dollars to spare, I recommend PSP X2, if you're budget concious, try the Gimp. PSP is also available as a free download (Ver 10 I believe, X2 = Ver 12) when you buy certain memory cards.