MS BckOffice Small Bus Server 4.5 {Insta

I am currently trying to install SBS 4.5 on my server, as it has been hit by the Nimda Virus. I have lost the boot disks and have made some from the cd. However, when i try to install it, it gets to the stage where it prompts you to specify other h/w and then proceeds to ask for boot disk #3. When you input this disk and hit ‘enter’ it does not recognise that it is disk #3. What am i doing wrong? Please help me out, urgently.