TradeKeys is a utility from PC Magazine that, although listed as free, requires you to take a subscription to access the file library. This can be for as low as around $5 and may be worth it if you use Windows NT, 2000, or XP and want to swap other than the Ctrl and Caps lock keys; on-line subscription details are displayed when you try to download the program from the PC Magazine Web site:
"TradeKeys 2 lets you map your keyboard almost any way you want. Under Windows 95, 98, and Me, you can change, swap, or disable any of the standard keys (those that existed on the original 84-key keyboard layout). Under Windows 2000 and XP, you can also remap extended keys such as Right Ctrl and Right Alt, the Windows logo key, and the navigation cluster. You can save your mappings to a file for reloading later. This lets different users on a system switch quickly between different mappings."
To find TradeKeys go to the PC Magazine Web site and type TradeKeys into the search box at the top of the pages on the site.