Summarize this article:

This is a freeware conversion program by Macroplant, which you can download from macroplant.com/adapter/. As long as you download from Macroplant, you won't need to worry about adware or browser toolbars. Adapter is completely free, and is available for Windows and Mac. You can also click the "Browse" button in the main Adapter window to find the file on your computer. You can add multiple files to the conversion queue. It may take a few moments to load longer WMV files. This will select the AVI preset for the conversion. This will open the Settings pane. The "Directory" section will allow you to select the location that the converted file will be saved to, as well as the file name. The only setting you may wish to change is the "Quality" setting. "Medium" is the default setting, which will result in a small file size but with some quality loss. If you want to keep the exact same quality as the original, select "Very High (Lossless)." This will result in a larger AVI file. You can also use the Settings pane to trim the video clip or add a text overlay. Your video file will be converted according to your settings. The time this takes will vary depending on the size of the original file, your quality settings, and the speed of your computer. You'll find the converted file in the location you selected, which is the same as the source WMV file by default.

Summary:
Download and install Adapter. Drag your WMV file into the Adapter window. Click the drop-down menu and select Video → General → Custom AVI. Click the Gear button. Select the output location and file name. Select the video quality you want from the "Resolution" section. Click "Convert" to begin the conversion.