The most basic thing you can use is plain ole Paint. I use the stretch/skew option in the image menu to resize by percentage - the attributes option in the same menu will show you the dimensions in pixels, and let you resize that way too, but it'll crop the pic rather than shrinking it, so you want to be careful with it.
Other than that there are lots of freeware/shareware image programs out there.
GIMP is good, and free, and not that hard to learn.