real player is useless, it can't handle high-bitrate encoded videos. there is no alternative player as far as I know. the only way is to convert it to other format (wmv, divx, mpeg etc).
you can convert rm/rmvb to wmv using
real alternative and
wmv encoder. both are free download.
eo-video is also pretty good but slow. it allows you to convert them to divx/mpeg format. it takes 3 hrs to encode an 1hr long episode to divx using eo-video on my athlon 2600+. it's a pain but I don't know any other ways to do it.