Should you need another OS emulated within your system, QEMU is the Free Software that do the job well.
In Ubuntu 6.06, you can have the version 0.8.0 packaged in the universe repo.
To use it, just install it using apt-get (or synaptic if you prefer):
$ sudo apt-get install qemu
then create the virtual hard drive using qemu-img
$ qemu-img create mydisk 3G
to install an OS into your emulated hard drive (using CD-ROM installer)
$ qemu -hda mydisk -cdrom /dev/cdrom -boot d
Qemu can be emulating at higher speed when it uses its accelerator module named Kqemu. However Kqemu is a proprietary software (although it'll cost 0 in price). To install the accelerator, you can use the method posted here