I have Ubuntu 10.04 installed in Virtualbox. The VB installation image is around 4gb(.vdi file). I dont wish to reinstall Ubuntu and then install everything all over again.
Can I boot natively using this image?
If you are using a recent version Windows as your host system...
You can convert the .vdi file to a .vhd file (this is a Microsoft virtual hard drive. You can convert the file using the command line VBoxmanage.exe tool. Virtualbox will quite happily use the vhd file if you replace the vdi file with the vhd file for the virtual machine).
(The TechNet link also explains how to install the Windows 7 bootloader on an older version of Windows.)