I struggled figuring out how to install UNR on VirtualBox... problem is the only thing that is provided with a UNR install is an image file only to be turned into a bootable USB stick/drive. VirtualBox does not support bootable USB devices. So i found this post... and I'll paste the intructions here as well, in case the link ever goes dead:

You'll have to convert the IMG file into a VirtualBox VDI file... then create a new virtual partition, and then create a slave drive, and load that newly created VDI file. Crazy!

To explain #3, to get to the location, I just put the IMG file on my desktop and typed "cd ~/Desktop" to locate my file. I just wanted to put this for Linux users who aren't versed in terminal commands.

http://blog.ibeentoubuntu.com/2009/05/playing-around-with-ubuntu-netbook.html

1. Install VirtualBox, either by downloading from Sun's site or by installing the FOSS version from the repository.
2. Download the .img file from the UNR download site. Use BitTorrent if you can -- it's faster and easier on the mirrors.
3. Convert the.img file into a VirtualBox .vdi file. In order to do that, you'll need to use VirtualBox's command line. Open a terminal and cd to the directory that you've downloaded the ubuntu-9.04-netbook-remix-i386.img file to. When there, issue the following command:
VBoxManage convertdd ubuntu-9.04-netbook-remix-i386.img ubuntu-9.04-netbook-remix.vdi
This will leave you with the original file and a new file, ubuntu-9.04-netbook-remix.vdi.
4. Create a new machine in VirtualBox with a 4GB hard disk, choosing "Linux" as the operating system and "Ubuntu" as the version. Give the machine between 512MB and 1GB RAM. Increase the video memory to 16MB. Finish and close the wizard.
5. Add the Netbook.vdi file as a second hard disk. Open the "Hard disk" tab of the machine's properties, and click the plus icon, setting the new drive as "Primary slave." Select the ubuntu-9.04-netbook-remix.vdi file, adding it with the Disk Manager when requested.
6. Boot the machine and hit F12 to choose to boot from the second hard disk. You'll need to press "2."
7. Choose your language and to "Install Ubuntu Netbook Remix." I won't explain how to do that here. You can give it the whole disk. It should be a simple, if slow, process.
8. Reboot the computer. You'll be in the UNR interface, but it will be dog slow because of the generic VESA driver being used. Luckily, there's a way around that problem.
9. Install the Guest Additions. While the machine is running, go to the machine's VB menu and choose Devices -> Install Guest Additions. If you are using the open source version of VirtualBox, you'll probably be prompted to download the Guest Additions .iso file, which will take a few minutes. The disk will appear in the right-hand menu bar. Click on it to make sure the CD is mounted. Open a terminal (in Accessories) and type the following:
cd /media/REACTOS
sudo ./VBoxLinuxAdditions-x86.run
10. Install updates. You should be prompted for this. I won't detail it.
11. Reboot and use the computer. Mine is still a little slower than it should be, but it's OK with the Guest Additions installed.