You shouldn't need to
buy any photo-processing software. There are plenty of good freebies around:
If you want something that's as good as the 'full' (mega-expensive) versions of Photoshop, look no further than GIMP:
http://www.gimp.org/
However (just like the full versions of Photoshop) it's got a really steep learning curve, making it too advanced for many people. (If you do decide to give it a go, make sure that you download the user manual as well as the program; otherwise you'll probably be left totally bewildered!).
Irfanview is probably best known as an excellent picture-viewing program (which is about a zillion times better than the basic one that comes with Windows and thoroughly deserves a place on every computer):
http://www.irfanview.com/
However if you 'look under the hood' you'll find that it's got some excellent image-editing functions as well. One of the great things about it is that you can use Photoshop plugins, such as SmartCurve (which I find invaluable):
http://www.softpedia.com/get/Multimedia/Graphic/Graphic-Plugins/SmartCurve.shtml
The only problem that Irfanview has (for most people) is that there's no lasso tool, so you're restricted to selecting rectangular areas, rather than being able to draw around the edge of an object.
Google's own Picasa is also popular, but I've not used it myself:
http://picasa.google.co.uk/
Ditto for Paint.NET:
http://www.getpaint.net/index.html
For many people though, the best freebie of the lot is Serif's excellent PhotoPlus Starter Edition:
http://www.serif.com/free-photo-editing-software/?MC=FSSPHOTOPLUS
It does just about everyone that most users require. (If you want to go a little bit further then just wait for the emails from Serif which will offer you the latest 'full' version, PhotoPlus X7, at half price of around £40 or [better] PhotoPlus X6 at a tenner including a proper printed manual).