Disk boot failure - insert system disk

I've read other threads on this problem, but nothing seems identical to mine.

My boot failure began posting intermittently a week ago (1 out of every 3 or 4 boots) and my work around was the Reset button. Now, the message occurs every time I boot. I cannot get to my Windows XP Pro because the "mirror" file I made with Nero is on a CD, but the computer won't boot from the CD.

The computer is about 4 years old. The boot drive is only 30M and pretty full, with a partition for the OS. The only significant thing I've done lately is run PC Registry Mechanic. There are no strange noises, and the floppy drive is empty.

I have Windows XP Home on another drive, which is a slave. Obviously, I could make that master and investigate, but surely there's a smarter way?

Thanks everybody.....this site is EXCELLENT