I have to say I'm getting a little confused at what you want to do.
In a previous post, you suggested Kubuntu is working fine, but you prefer Mepis. Now you're talking about PCLinuxOS.
If you disconnect the Vista drive, it will at least protect it that so no harm will come to it. Installing (whatever version of) Linux to your XP drive is perfectly feasible, but as you wont have Vista connected, it cannot recognise it to set up a dual-boot system. That can however be added later, but really you could do with gaining some experience before attempting it.
Have you read my article on installing PClinuxOS? It shows you how to select the drive you want to install to: click here
Take a look at this as well: click here as it will explain how Linux identifies hard drives.
Hope that helps.