Install CD required for boot

I have just re-installed WINDOWS ME in a new hard drive using the OEM (Dell)
system CD. After the initial installation, I removed the system CD to
install some other drivers. After the subsequent "restart your computer"
instruction, I found a text message on the monitor that it was an invalid
boot disk and to insert a boot disk in the floppy drive. After puzzling over
this I realized I had removed the system installation CD. When I reinserted
the system installation CD, I then got a two line screen asking if I wanted
to boot from the CD or the Hard Drive. There was a time out and it defaulted
to the hard drive. Either way, selecting the Hard Drive resulted in the ME
system starting normally. So it now appears that every time the ME system is
booted, it must see the system installation CD in the CD drive to make the
choice of CD or hard drive, otherwise it gives the "invalid boot disk" error.
I went through the whole install procedure again, with the same result.
This is a friend's system, and I have not had any experience in installing an
ME system. Any ideas as to why this is happening?
Obviously, keeping the install disk around for every time the system is
booted is a workaround, but this should not be.

Don Robertson