Installing Windows XP from an ISO image on modern versions with sane mouse pointer and networking support:
qemu-system-i386 -localtime -cdrom /vagrant/win.iso -m 384 -boot d windows.qcow -vnc :1 -usb -usbdevice tablet -net nic -net user
My personal invocation of QEMU on a Fedora Core 4 box (2.4GHz, 1GB RAM) for a Windows XP virtual machine (3.8GB disk in a qcow compressible image, containing a replica of my corporate environment) was this:
/usr/local/bin/qemu -hda xp.qcow -user-net -k pt -m 512 -redir tcp:3389::3389 -nographic -localtime
rdesktop -0 -a 16 -k pt localhost:3389 -g 1280x1000 -D -P -K
The above set the Remote Desktop window to take up the whole screen minus the 24 pixels taken up by the Gnome bar, and allowed me to use Ctrl-Shift (Arrows) to instantly switch between virtual desktops.