I'm not sure that's a mishap.

Can you boot into the new 80GB XP from the multiboot menu? then do that.

Transfer anything you want from the smaller drive to whatever folders you want to on the new drive.

Put the XP install cd in the drive (hold down shift so it doesn't autorun. If it does just exit out).

Reboot the computer. When the screen says "press any key to boot to the cd," press the space bar. This assumes you have your BIOS set to boot to cd first. If you're not sure, instead press the key to enter your BIOS as soon as you see your BIOS screen when you reboot. On mine it's the delete key. Sometimes it's F1. Once there find the section where you have the bootup order and set it to boot to cd first. Save and reboot again. Boot to the XP cd.

When it asks, press R to enter the Recovery Console. It will probably have 2 Windows to enter listed. Choose the one that's on your new drive (probably D:Windows if you left your original hard drive where it was and just installed the new one). In other words, press 2, then the enter key. For Administrator, usually there's no password unless you went and booted to SafeMode and gave the Administrator Account one. It doesn't mean the one with your name that XP set up when you installed. So, just hit enter. You'll see D:\Windows as the prompt (assuming that's the drive). Type "format: c: /Q/FS:NTFS" without the quotes. When you reboot choose to start from the cd again and reenter the Recovery Console. After getting to the D:\Windows prompt again, type "FIXBOOT" without the quotes. Reboot to normal Windows, and the old hard drive should now be a usable empty drive. Check it in My Computer.

I am somewhat knowledgable, but still just a computer hobbyist. The above is what I would try, but I also must make clear that this stuff is VERY DANGEROUS and you could lose everything! So, while you're in the new install drive transferring files make sure you BACK UP anything you must not risk losing to cdr's.

If it all becomes fubar'd by this, I would suggest formating both drives, physically moving your new drive to the Primary IDE port and removing the now formatted old drive. Install XP. Install your Motherboard chipset drivers. When you're ready, physically install the formatted old drive to the Secondary IDE port. Now you'll have the situation you described that you wanted: The new drive as your main drive and the old drive as more space for stuff.

I think my steps should work. To be safe however, I would do some more research on your own and see for yourself what is sensible. It's your machine and your files. BACKUP, whatever you do.