In addition to posting that /var/log/vbox-install.log, it would help to know what version of virtualbox you are trying to install.
There are several "virtualbox" packages. For instance if you have enabled the download.virtualbox.org repo you will find seven "virtualbox" packages in Synaptic (virtualbox, virtualbox-2.0, virtualbox-2.1, virtualbox-2.2, virtualbox-3.0, virtualbox-3.1, virtualbox-3.2). The latest "virtualbox" is the virtualbox-3.2 package.
If you've tried to install the earliest virtualbox (the package named just plain "virtualbox"), the kernel module may fail to build because Mepis 8.0's 2.6.27 kernel might not be supported. I know this is the case when trying to install the "virtualbox" package in Mepis 8.5 and it's 2.6.32 kernel, the kernel module fails to build. With the "virtualbox-3.2" package, the kernel module builds just fine. I have a similar result in Mepis 8.5 with the Debian Lenny virtualbox-ose package.