If you come across a problem where you can't do anything but install a Windows XP alongside a Vista or Windows 7, don't despair, as it is quite easily done.

First you have to repartition your HD. If you have a second HD you're not using, you can ignore this step. If not, you probably have to resize one of the present partitions. But in order to get the maximum out of the resizing, make sure the partition you're about to resize, is defragmented properly. Note that you cannot resize FAT32 partitions (but who uses that nowadays, anyway).
The resizing can be done through Disk Management. Select the system or another partition, right-click and select Shrink volume. Windows   ...