A chap at the Oswestry dog show on Sunday was telling Jimmy and I about how wonderful
Picasa is. Whilst he may have been an uneducated canine licker it is a good (and free) utility for basic enhancements. Other decent free ones are
Irfanview and my favourite
Fastone Image Viewer.
The Gimp as already mentioned is a powerful (and again free) Photoshop alternative but can be a little intimidating to get to grips with at first.
At least one of these should suit your needs.