I don't think a Windows 2008 driver will be available - this product has been EOL'd for a long time, maybe since end of 2006 if I remember correctly. You may either disable onboard RAID and use native SATA mode, or use an add-on RAID card with Windows 2008 support.
First, understand that the platform you are using is end-of-lifed and has *never* been tested with Windows 2008. This is an unsupported config and the driver you are using is a Windows 2003 driver *only*. It won't work with 2008.
If you want a RAID1 for redundancy, your best bet is to just install Windows 2008 to the SATA HDD in AHCI mode, and then on the first boot initialize your 2nd HDD and use the OS-level RAID1/mirror function.
Sorry for the bad news.