>I installed Vista on a laptop that was little used at the time and now want to go back to XP. The laptop is now in regular use and we are having trouble getting it to work amiably with the other computers in the house, both of which are running XP. There doesn't seem to be anything built into Vista that will let you revert to XP, which it was running before. I tried booting from an XP Pro installation CD but the setup program was blocked by Vista. I even tried formatting the laptop's C: drive. No luck.
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>Is there any way to do this?

I did it with a bootable CD with XP SP1 - and it allowed me to do it. The necessary step before it was to create a partition for it, which luckily is possible to do in the Vista. So I just installed it into the new partition, not stepping over Vista's toes, and it didn't complain. Didn't want to boot afterwards, either.

If you can't do that, you may burn a DVD with Knoppix, it's bootable by default, and it should allow you to repartition the HD any way you like. With fresh partitions, the disk should be blank - then you should be able to install whatever you want.