This is actually quiet easy to install. This is how I done it for any one who is interested.
1) Download VirtualBox 1.5.0 from here http://www.virtualbox.org/wiki/Downloads
*** Choose the binary for "All distributions" ***********
32-bit download here http://www.virtualbox.org/download/1.5. ... ux_x86.run
64-bit download here http://www.virtualbox.org/download/1.5. ... _amd64.run
2) Right click on the downloaded "run" file and change it to an executable.
3) Open up a konsole and change directory to where you downloaded the "run" file.
4) Type "su
" and enter root password
5) Remove old vboxdrv
kernel module from memory (if it's loaded) by typing "rmmod vboxdrv
" (without quotes).
6) Un-emerge your existing Virtualbox. E.g emerge -c virtualbox-bin virtualbox-modules
7) In konsole type "./VirtualBox_1.5.0_Linux_amd64.run install /opt/Virtualbox
" (without quotes and replace with your downloaded file name)
A start icon should be placed under kmenu->applications->system->Virtual Machine(virtualbox)
note: if it comes up with an error when you run it then try to reboot first.
All the best
p.s. Here's a list of changes http://www.virtualbox.org/wiki/Changelog
It now supports 64-bit windows as guest OS.