Hi Mr.mick-duck,
I have had some issues with Virtual box myself.....
Q1 Did you install virtual box by selecting it from the install options within Mandriva 2008.0?
You may be able to work around the issues by deselecting the current installed version, & obtain the following version from the link below;
http://www.virtualbox.org/wiki/Downloads
Select Binaries (All Platforms) and follow the links to the required platform version in your case it will be mandriva 2008.0.
I hope this is of help.
Jnike