When you set up XP, it will go to a screen and what it will come to is showing you what you have already. It will list your drives\OS and ask which one you would like to use. So you know the one you are installing has no OS, that would be the one you want to format and install to. You can do a quick format, that is fine and of course it will NTFS format. Then it will install to that drive.
Typically when you reboot, you will get a black screen asking which OS you would like to boot to, it's pretty straight forward. There is nothing different except you choosing which OS to load. You use the up\down arrow keys and hit enter. If you don't, eventually it boots to the first listed OS which should be Vista. Not too bad huh?
Of course there can always be problems but that comes with the territory , plus you are attempting this on an Emachines OEM.
Don't forget, if it won't boot from CD, you may have to set it in your bios and set it back after install.
Password the dual booting? You would have to do that in bios once again. Also, this will vary between manufacturer as to IF and how it's set up. Also, be warned, you pass protect in bios, if you lose\forget password, most often you have to remove the Cmos battery or use jumpers to reset.
I have checked out various threads on this subject at CNET and I can follow most of what has been said. I have an e-machine that came with Vista. I have since purchased a new hard drive with the thought that I would interchange hard drives using XP on one and Vista on the other. But now, I have decided that I would like to have both drives in at the same time and use the dual booting feature. The main drive would be Vista and the slave drive would be for XP. I know how to install the new hard drive, so that is not a problem. Where I am uncertain is what happens down the road, so to speak. When I insert the CD for XP and it asks where I want it to be installed, I gather I should indicate the secondary drive. But what I haven't found out is what happens after this. That is, when I reboot my computer, just what comes up on the screen and at what point do I need to indicate which OS I want to be using. And even more importantly, how do I switch back and forth between the two systems. Also, as a side issue, is there anyway to password protect the dual booting so that only someone with the password can access the XP operating system.
The Ancient One