Installing Mac OS X on Non Apple Hardware
How can I install OSX Snow Leopard and Windows XP 32-bit on same HDD?
Use Boot Camp. It is installed with Snow Leopard and can be found in
Boot Camp will create a second partition on your hard drive then assist you in installing Windows on it. You can then dual-boot the machine.
In addition, I believe Parallels and VMWare Fusion will allow you to run the Boot Camp partition in a virtual machine. This can be quite handy when you only need a Windows program for a minute and don't want to have to reboot into the Boot Camp partition. I saw an article the other day showing that Parallels was quite a bit faster than Fusion, fyi. I use VirtualBox b/c it is free though, and it is great, but it can't virtualize the Boot Camp partition.