For a very basic, but quick to download and also free (to non-commercial users) I use Irfanview.

Like I say quite basic but can be used in Windows 9x, ME, NT, 2000, XP, 2003. Very quick to download so if you need a viewer when away from home this is a good one to download.

Allows resizing and rescaling of images, and has some effects to jazz up images!!