QEMU, is a great alternative to VMWare or XEN, for installing let's say Windows on your Linux Machine
First get the QEMU RPM or use YUM :)
yum install qemu
Now create the directory where you are going to install your VM
cd $HOME mkdir XP cd XP
After that create the file that will content your guest OS
qemu-img create xp.img 8000M
Here the 8000 Mbytes is what you will have available as disk space for XP, you can change this to fit your needs, (and your disk space)
Now boot your new machine, I am using the winxp.iso which the ISO file of the instalation disk of XP.
qemu -boot d -hda xp.img -cdrom $HOME/winxp.iso -m 256 -localtime
if you want to use the CDROM, you may enter.
qemu -boot d -hda xp.img -cdrom /dev/cdrom -m 256 -localtime
the -m parameter indicates the memory for this virtual machine, use with care and do not allow the guest OS to use more than the half of your total memory, better if even less. (default is 128M
From Here the instalation will begin as normal, then when you finish, each time you need windows, enter.
qemu -boot c -hda xp.img -m 256 -localtime
That is all, hope it helps.!!