Depends on what you REALLY want.

I agree with the person who suggested Irfanview for the most basic photo editing tasks. It's a good, solid, capable program that is primarily a photo viewer (the best photo viewer, in my opinion), with editing added sort of as an afterthought. I have had my photo files default open in Irfanview for years because it is the fastest and most tailorable viewer out there.

But if you actually want to get into a little bit of actual photo editing (what is generally referred to as "Photoshopping" your pictures) and don't want to pay through the nose, then I recommend the freeware open-source GIMP. It's been around for years and years and years, with programmers around the world continuously tweaking it, and is very nearly as capable as Photoshop itself. Indeed, it comes with a set of filters that even Photoshop can't match.

I use Photoshop and Paintshop Pro (Paintshop Pro is particularly tailored to optimizing digital photos and has very useful batch functions) and GIMP.