I found a post with instructions to install xp on a laptop with vista already installed. Followed the directions to the letter and got a message at the end that said I couldn't partition the hard drive because it already had all the partitions it could take. Why did I think this was going to be easy?

U r having Vista so it must be in C:\ drive means u have already partitioned the Harddisk so need to do it again. Just insert XP cd goto D:\ drive just install it. If u want u can format (recomended) D:\ drive using FAT 32 or NTFS file system. If u want more on any contact i will try my best to solve it.

You suggest installing xp on D: and formatting the D: but the D: drive is already set to be the recovery disk.

You likely have all of your hard drive space taken up by the two existing partitions C: and D:
You need to resize (make smaller) your partition containing Vista, C:

You'll need a third party non destructive partition utility like GParted.
GParted -- Welcome

Resize your Vista partition, smaller by the amount of room you need for the new OS. Then create a new partition in the now empty space left on the hard drive.