You can convert it to .avi by renaming it with a .avi extension.; But that wont do you any good as .RAR is not a video format; its an archive format. Download either WinRaR or 7Zip (FREE) and extract the files within.
RAR files are not video files, theyre archive files, just like .zip. Thus, a RAR archive _can_ contain a video within itself, but you need to extract the RAR archive first.
Load 7-zip or WinRAR and open the rar file. Then, extract the contents (files) inside that RAR file into some folder on your HDD. If the files "found" inside the RAR file are in AVI format, youve achieved your goal. If theyre in some other format (say, .3gp, .mp4 or something else), you obviously need to convert the files further. For that, you might want to check AfterDawns video conversion guides.
I think I already read a similar one to this...how to convert unknown file to avi, but Im not sure if this is also applicable to convert rar to .avi extension. try it and see if it works.