Matt--try this free program: http://www.irfanview.com/
it's small and has some nice resizing features. Very intuitive, too, with a short learning curve (if there's any at all). You can resize to specific total pixel sizes, by percentage, etc.
It worked in XP. I haven't put it on my Vista laptop yet, but it's not so complex that Vista would likely not work with it.
There's things it can't do, but it handles most basic editing needs.