I had a similar problem when I last installed XP on a P5NSLI. It couldn't see my SATA drive so couldn't get XP loaded onto it. It would just hang.
I had to specify an additional driver during the initial stage of the install. The driver was contained in the unzipped file 15.25_nforce_winxp32_international_whql.exe . You can get this from the nvidia website.
Of course you'll want the Windows 7 version, and there is probably a version higher than 15.25 now.
For my XP install I specified the driver contained in 15.25_nforce_winxp32_international_whql\IDE\WinXP\sata_ide