Boot with the DVD .
Press F8 then type -s.

If you only have one disk it's named rdisk0, if you have two rdisk0 and rdisk1, if you have three ... etc

type fdisk -e /dev/rdisk0
type print (it will show you each partition with their type)
type f X (where X is the Windows partition number, to make Windows partition active)
type update (to update the machine code)
type w (to write the change)
type q (to quit)

then reboot.

If all goes well you should be able to boot in Windows.

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Mac OS X 10.6.3 • 2.8 GHz Intel Core I7 860 • ASUS P7P55D • 2 GB 2 Ghz DDR3 • ATI RADEON XFX 4890 1GB • 2 x Hitachi 160 GB Serial-ATA • PIONEER DVD-RW DVR-112 • LaCie Desktop Harddrive 750 GB USB