This can't be done (to my knowledge) in one easy step, but there are several ways to go about this problem. It all depends on how much free drive spave you've got.
First I have to say that a reinstallation of Windows XP isn't such a bad solution. It'll all be clean and fast again, but there is that hassle of reinstalling all you programs again.
If you don't really want to reinstall this is the way I would go:
First install the 6Gb harddisk as the primary (Master), and install Window98 on it in a folder name nything but Windows, Win98 might be nice. Just do the most basic install, you only need it to copy your XP files. Now start a fresh installation of Windows XP again on the 6Gb drive, and choose to leave Windows98 on it, creating a multiboot system. As soon as the machine wants to reboot for the first time, for starten Windows XP in GUI mode make sure you be quick enough to start Windows98 instead. All this did was install the Windows XP Bootloader, and thats all you want. Now delete the two folders the installation add to the 6Gb drive.
Now add your 20 Gb harddisk as a secondary (Slave) and see if you can see the files on that drive from Wndows98. This depends wether the 20Gb drive was formatted as Fat32 or NTFS. If it was NTFS then install "NTFS for Win98" so you can see all the files on the 20 Gb drive.
Now copy the Windows, Program Files, Documents and Settings, and System Volume Information (if that exists) from the 20 Gb harddrive to teh 6 Gb harddrive. Merge the boot.ini from the two drives, but delete the line that wants to install Wndows XP.
By now you should be done. Don't delete the folders from your 20 Gb harddisk untill your absolutly sure your installation on the 6 Gb drive functions completely.
I'm sure I'm forgetting something, just let me know what goes wrong and we'll look further. And remember, just change the 20 Gb drive back to Primary (Master) and everything is back as it was, so it can't fail!
Good luck, let me know how things go. Oh, and yeah, I know, there are lots of other ways, but since noone replied yet, this would me my way, but if anyone knows anyother way, let us know
