Flash Video Content (flv) is a video format defined by Adobe Systems and is the standard video format used by almost all embedded video on the internet- including that on YouTube, Google video and many more websites.
By converting flv files to avi, you increase the number of applications on which you will be able to view the files.
There are a number of applications you can use to carry out this conversion, one of the best (and free) is Pazera, which you can download here.
However, if you are just looking for a suitable media player to use to play the file, an excellent choice would be VLC, a free, open source media player which plays practically all file types and runs on all platforms. Definitely worth a download!