Boot freeze with SATA connected to vt6421a PCI card.

Hello, and thanks for looking.

I've just installed a generic VT6421A PCI card, which gives me a spare IDE connection and 2 SATA connections.
I've booted up into Windows (XP Pro, genuine & fully updated) and installed the drivers.

I connected the SATA drive (jumper set to limit transfer speeds to 1.5G, as the card does not support SATA2) in its external casing to the card. Windows freezes for periods of around 10 seconds, with only a fraction of a second between times to move the mouse. The freezing stops when I remove the drive.
The drive does not appear in "My Computer", but it will show up in the Device Manager as a SCSI drive.
This behaviour I can understand, as I have not been lead to believe that the card allows drives to be connected after booting.

Here's where the trouble really starts.
When I boot with the drive connected, the system freezes on the black screen after the BIOS.

The drives external enclosure allows USB2 connectivity, which works fine, but again, the system will not boot if connected to the USB port on startup.

I've done a fair bit of Googling, and it seems to be quite a common (and unsolved) problem, so I'm thinking it may be a compatibility issue.

Would flashing the BIOS have any effect, considering that the motherboard is pre-SATA and therefore doesn't have any options in the BIOS relating to SATA?

Thanks in advance or any help or advice!
P