You cannot convert lone .ifo file to WMV (Windows Media Video) format. However, you should be able to convert whole DVD media (.ifo, .vob etc.) to a single video format (like the WMV), with some dedicated DVD Ripper software specially designed for this job.
hat it does is that it grabs and convert video data and saves them in one of the multimedia files, often compressed. This may lower the quality compared to the original.