"Edit to say tried Win XP reinstall again and now the stup program says there are no hard drives installed - even though the whole system was up and running fine yesterday"
I suspect that when you attempt to boot from the XP CD there is at some point a question - do you want to load other drivers? answer is yes, the driver for the raid array. That may be why it cannot see your HDD's. Is the raid array driven from a card or motherboard? Mine is card type and XP requires the driver to see the disks when booting from XP CD.
I had similar problem when a file corrupted and XP CD could only see an old installation on my backup (IDE) HDD's until I loaded the raid driver.
There are articles on the knowledge base that deal with boot.ini which will help you, they may well be the ones linked above.