Windows 7 boots only with installation disk in the drive

Windows 7 boots only with installation disk in the drive

I have 2 hard drives - one on SATA and one on standard ATA. On the SATA drive I have Windows 7. I decided to do a fresh install on the ATA drive. Everything is ok apart from the fact, that the new install is booting only when the installation DVD is in the drive. When it's not, it boots the old installation on SATA drive. Does anyone know why it can be? My motherboard is Asus m2n-e.