Simply download whatever program that you’re trying to install (Home free edition for instance http://www.avast.com/free-antivirus-download) to your desktop or some other place that you can find easily.
BE SURE to remove any previous A/V products, and to use the removal tool for the product that was used. For instance, if you used McAfee, uninstall it from control panel > Add/Remove Programs, then download and use the removal tool from McAfee’s website: http://service.mcafee.com/FAQDocument.aspx?id=TS100507
Once complete (you should probably restart after the removal is complete), double-click the Avast installation file, and follow the on-screen instructions.