Vista, its got built in .zip opening-ing
same with window7
peazip its freenot Winrar you have to pay for it(form what i know)
Or 7zip which is the best. But if you're just working with .zips, you can open the zip file like a folder/right click to extract on windows 2000(I think) up to windows 7.
Or Gzip. Whichever you think is the best.