I had the same problem with my first attempts to install. Through the process of researching and plain old trial and error I discovered that it has some thing to do with my specific Hard Drive it doesnt read Darwin in the mbr. (I read somewhere that Darwin is written in Hex and my Hard drive is mbr is only capable of Binary, but I dont know how reliable that information is). I was able to fix it by booting with the DVD then using the terminal to copy the files from the DVD in /usr/standalone/i386 and placing them on a usb thumbdrive. Then installing Windows and pasting those files previously copyed on the root of the C:\ drive.
I have read here
http://wakeuppakistan.blogspot.com/2006/02...and-win-xp.html that you can just edit the boot.ini file to include
C:\chain0="Mac OS X x86"
However, I also boot Linux so I used Grub to boot. but the same idea applys. I am not aware of any wayto boot JUST Mac OS if your hard drive makes you put in the DVD to load darwin.
That was my fix hope it helps.