I have a Hewlett Packard Pavilion 6740C (Windows 98 SE) with the original 15.0GB hard drive installed as the master drive and a Seagate model ST340810A 40GB hard drive installed as the slave. I have tried to install another Seagate model ST340810A 40GB hard drive as the master drive but get a message that there is no operating system installed even after I put the OEM CD into the CD drive to install the operating system. I don't know what to do now to get the computer to let me install the hard drive and then install the operating system on the new hard drive with the OEM CD.
Can anyone please help me?

It sounds like the bios is set up to boot from the hard drive even if a CD is inserted. What you want to change is called the boot order (or similar). You'll want to make the CD be the first thing, then the hard disk. Now, when you boot, it will start the CD and prompt you to install Windows.

To get into the bios, you'll probably hit F1 or F10 during system startup. It may even say which key on the screen to hit, but it only stays on there for about a second, so you have to be fast!

Once in the bios, look for Boot Order (or whatever) and change it as indicated above. Be sure to save settings and exit. If the CD is in, you should hear it spin up and you'll get a Windows setup screen.

Good luck,
WallyH

P.S. Make sure you have the product key handy when installing. If you system is still running now, you can download the Magical Jelly Bean Keyfinder (http://www.magicaljellybean.com/keyfinder/) to retrieve your product key.