Ok, not so easy then. I would download Easeus free partiton manager
http://www.partition-tool.com/personal.htm and shorten any existing partition(s) as required to release at least 20Gb free space. Create new primary partition and format NTFS in the free space.
Either take the PC to another PC, connect broadband and copy the XP install CD onto the new partition into an \xpinstall directory. (only part needed as the post you quote says).
Alternatively, extract the HDD and connect to another PC with an adapter (very cheap) and again do the copy as described.
All pretty much the same as resulting from the post you quote.
Then booting the laptop starting a Dos window and away you go, except you are already on your laptop, so no need to halt the process part way through. You may have trouble if the laptop only has SATA drive. You need sata drivers during XP install in that case.
After a successful install laptop will then only boot into XP. Your existing Vista is still there, but will be inaccessible until you manage to re-write the boot manager again, to include Vista and XP as dual-boot. Also not easy without an external drive. Burning or buying a copy of UBCD4WIN might help if you had.
BTW which part of the instructions by thoutcaste do you trip up on? To be honest I cannot see much hope of you solving the setting up of dual-boot problem without an external drive, but I might be corrected on that.