I cannot install RealPlayer properly

I have Windows XP Home Edition (Service Pack 2Installed). I have downloaded
RealPlayer (which is free) but could not install it properly. In the program
menu it shows but it does not get started. Even it could not be uninstalled
as it shows that uninstaller is unavailable!!!.
I have Windows media player running perfectly.
The error message is - Shell executable program has encountered an unknown
error. So it must close
Can anyone help??
Himadri