I'm still using an ancient (Win 3.1) shareware version of a program called LView Pro. It's certainly no substitute for Photoshop, but you can play with gamma, color balance, do resizing, convert image file types, and use some different image filters to smooth or sharpen an image. It doesn't support some of the newer JPEG file types or animated GIFs but it still works on most images. It probably won't work on any OS newer than WinXP (just my guess). Let me know if you are interested and I'll see if I can send it to you.
Or, Google is your friend, I bet there are lots of more current image software out there. I think I have heard good things about IRFANVIEW but I have only used it very briefly...more info here:
http://www.irfanview.com/