SATA config

I have just installed a 120gb SATA drive and installed Win XP Pro on it without a problem. However, the problem is I had to buy a SATA PCI host adapter to use it. The power supply has a SATA power adapter with it so that is not a problem. When I try to use my normal ATA Seagate 80gb HDD with it, I can’t boot into windows, I get the message invalid system disk. Seems to me it is trying to boot to the ATA drive not the SATA drive. How can I get around this problem?