Irfanview should be on everyone's PC, simply because it's the best image
viewing program available. However it's also got some good image
manipulation features:
http://www.irfanview.com/
Picasa (from Google) is very popular, and easy to use:
http://picasa.google.com/
GIMP might be rather advanced for your needs,but I'll mention it because it's the nearest that any free program comes to matching Photoshop:
http://www.gimp.org/
For casual users, such as yourself, the star program (among the freebies) is probably PhotoPlus SE:
http://www.freeserifsoftware.com/software/Phot oPlus/default.asp
Chris