Once you remove Vista by deletion of the primary partition on the drive there you void the warranty. You know that don't you? You would need a drive partitioning tool to see the Vista primary and likely the hidden recovery partition totally removed. The XP installer can't do that due to Vista being the newer version of Windows there.
HP's support site would be he place to look for updates on that model for XP before trying this. Those should be seen since Vista is still the new OS out from Microsoft. After verifying those will be available you then simply reformat the Vista primary if not deleted entirely for a new one and install XP onto that.