adding raid controller to system with onboard raid

I currently have a KG7-raid abit board which has the highpoint raid
onboard. However this only support ata/100 . I have been trying to
add a disk to my system but have had problem with it working. It is a
newer ata133.

I've tried falshing the bios and updated the driver to match etc but
no luck.

The system currently has 2 disk (ata/100) sitting on the onboard raid
controller which are striped . This is my XP operating system.

When I put the raid controller in sil 680 ata/133 raid controller the
system can't see the 2 disks sitting on the onboard raid and thus
won't boot.

I wasn't actually going to use the raid functions on the new
controller card just use them as IDE ports .

Can I do this with my setup or will I have to put the 2 disks on the
new controller and re-install XP( don't want to do this as I have a
lot of stuff on there !)

Please advise