how are you installing? ide or sata or usb dvd drive? what kind of motherboard? how is your hd(s) setup (partitioned)? all of the HD to SL or ? MBR or GUID? some specifics will go a long way in us being able to help you.

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[HACK P5K]
ASUS P5K Deluxe/Wifi [ Q9650 / 8GB OCZ / EVGA 8800 GTX ]
Windows Se7en 500GB SATA
OSX 10.6.4 (vanilla-EFI) 500GB SATA
[HACK EP43]
Gigabyte EP43-DS3L [ E6700 / 8GB OCZ / PNY 250 GTS ]
Windows Vista 64bit 250 SATA
OSX 10.6.4 (vanilla-EFI) 160GB SATA