you must install qemu first ... run this :

sudo apt-get install qemu

then create a virtual filesystem (to simulate a hdd) (1000000 = 10go), under your home directory

dd of=hd.img bs=1024 seek=1000000 count=0

put a bootable cd (or an "iso-file") in your cd drive (win2k, xp, linux live-cd, ...), and run

qemu -hda /home/[your_name]/hd.img -cdrom /dev/hdd -boot d -m 512 -user-net

option -hda = the virtual disk to use (~/hd.img)
option -m = memory to use (512 mo).
option -cdrom = cd-drive (/dev/hdd for me) (or an iso file)
option -boot = which drive to boot (d as the cdrom)
option -user-net : net user mode (see http://fabrice.bellard.free.fr/qemu/qemu-doc.html#SEC21)

and it's run like a charm ...impressive