Don't even think about it!
Do you need both OS's. How big is your E: drive? Vista can use up a lot of space with restore points.
You can turn these off to gain space after restarting you can turn them back on and limit the amount of space they use.
If you don't need XP anymore one option would be to upgrade it to Vista and delete the Vista instalation off E: drive.
You could try shrinking the XP partition with Vista and try to add that space to the Vista partition, but there are risks involved in messing with partitions, so you would need to backup what you can't loose.
The best way to go if you can afford it, is add another drive.