Software is no problem...but
Software is no problem, Linux in it's various flavors is free and easy to use. You can find everything from Desktop to Server OS and applications , all you can think of. Even virtual machines etc. The problem would be to get cheap hardware. There are companies who collect old stock from data-centers for scrapping. Many of those bits for the scrapheap can still be used and are sold for a fraction of the price.
PCs that are not strong enough for an upgrade to XP/Vista/ W7 can still quite happily run Ubuntu even with little memory installed.