Error launching VirtualBox

When I try to start VirtualBox in 12.3 I receive the following error:

Effective UID is not root (euid=1000 egid=100 uid=1000 gid=100) (rc=-10)

Please try reinstalling VirtualBox.

i can get around this by running kdesu VirtualBox but this is obviously a bug.

You do not give any indication of what versions of kernel and VBox – repositories etc.

So this is a guess.
You have updted the kernel to 3.7.9, but are trying to use a VBox rpm built for the 3.7.6 kernel.
The way to avoid these problems (especially when using a non-stock kernel, or just after an update) is to use the Oracle rpms, which build the kernel modules against the running kernel. To do this you will need to install kernel-desktop-devel, kernel-devel, and the “Linux Kernel Development” pattern.

I am successfully running VirtualBox-4.2.8 on 12.3RC1 with the 3.7.9 and 3.8.0 kernels.

You can get the required files for this version of VBox and an x86_64 processor with:

wget -Nc http://download.vlbox/4.2.8/Oracle_VM_VirtualBox_Extension_Pack-4.2.8-83876.vbox-extpack