I can't play the *.rmvb clip normally without losting frames...HELP PLEASE~

I have a file *.rmvb and it’s playing 697.7Kbps (with 25.0 frames/sec).
I just found out that it lost frames frequently when I was playing it with Real Player 8 Plus.
Is it that the *.rmvb file should by played by RealOne player then it can play normally without losing frames?
Or is it that my computer is now fast enough to play such a high-quality file with nearly 700kbps?
My computer is Pentium III 450
with 256 RAM

What should I do to play the file without any frames lost?