You can uninstall most programs by opening Control Panel and then clicking Uninstall a program (under Programs). Oddly, not all programs are listed there.

MyUninstaller is a free tool for removing software that might not be listed. I needed it because I was trying to install Office 2007 and it gave me an awful error message about having needing to uninstall a beta version of Office 2007–but none were installed, and none of the software I needed to remove was showing up in the Uninstall A Program window. MyUninstaller did list some incompatible components (I have no idea how it got installed) and launched the uninstaller for me.
Thanks to Scott Hanselman for pointing me to this tool.