I have 3 hard drives; main C drive with Win7 already installed, a 1tb that holds games and stuff, and an 80gb that's really just sitting there. And I was wondering if installing Windows XP on the 80 gig would tamper with the Win 7 drive. I got the idea of just unplugging all the other drives for the xp installation, but I still want to be on the safe side and get a second opinion first.
Disconnect all drives but the one you wish to install XP on, Install XP, once it is installed set up your user account and get to the desktop, then shut down and connect other drives, boot up, enter the bios before Windows loads and set the W7 hard drive as the first boot device in the boot priority list. Save changes and exit the bios.
When the system restarts W7 should load if you got the bios set right, once you are in W7, Download and install this free software use it to Add XP to the W7 bootloader, once this is done and you reboot, you should get a nice bootloader to choose which OS you want to boot into. To get the free software scroll down the page in the link I provided, click "register" in the non commercial section, then on the page that loads just click download, no need to provide name or email.
When the EasyBCD program starts, go to the "Add New Entry" tab, select XP from the drop down menu in the Top Pane, be sure auto detect drive is checked. Hit the Add Entry button, then go to the "Edit Boot Menu" section and set which OS is the default, this means which one will load after 30 seconds if you don't make a choice when booting the PC.
This only applies if you install XP to a separate hard drive: If you do not wish to alter the XP bootloader with this software, you can just use the bios to select which hard drive to boot from, this is a bit less convenient but this method will leave the XP bootloader unchanged. The EasyBCD software changes the way XP uses it bootloader, is disables the XP boot.ini bootloader and installs a W7 style bootloader.
Similarly if you install XP on the same hard drive as W7 (separate partition) you will find only XP will boot now, follow these instructions
If you want to go with dual booting then by unplugging 1st and 2nd drive you will just have clean installation of XP and Windows 7 next to it. None of the systems will know about each other so basically dual boot won't work. You will have to switch bios settings to boot once from drive E(xp) and one from C(7).
Dual booting works quite well and is easy when doing it in proper order (Windows XP first, Windows 7 last). I've done multiple installs like that with Windows Xp, Vista32, Vista64, Windows7 32 and Windows 7 64 (in that order). It works out of the box in terms of you put cd in install, you put another cd in install another system on next drive etc.
When you get to the point where you have to do Windows 7 first and Windows XP later it's a bit tricky since Windows XP boot is not aware of Windows 7 existance so most likely you will have to fixboot/fixmbr from Windows 7 DVD after doing installation of Windows XP to fix boot menu. Otherwise you will have just XP working and Windows 7 won't be reachable.
This seems to be a nice article at Microsoft how to resolve post Windows XP installation where Windows 7 or Vista are already installed.
Couple of links to read up: