Actually, it's quite simple.
Step 1: backup everything you might want to keep. External harddisks or a networked computer are great for this.
Step 2: Download the mainboard and network XP64 drivers for your system. Usually available from your mainboard manufactuerers' site. Burn the drivers to CD.
Step 3: You'll probably have to use textmode drivers for your harddisk(s) during setup. If you have a floppy drive, make those floppies from the drivers you downloaded from the mainboard manufacturer's site.
If not, you'll have to make a slipstreamed XP64 installation CD. Download nLite (I don't have the link handy, but it's easy enough to find using Google), install it on your Vista machine, and build a new ISO image that includes the textmode drivers. nLite is very user friendly (as good as self-explanatory, actually), so that won't be a problem.
Burn the ISO to a CD, and use that CD to boot from.

Step 4: Insert the XP64 install CD (either the original or the slipstreamed version you made in step 3), reboot the PC. Use the F6 key if you have a floppy with a textmode driver, skip the F6 key if you're using a slipstreamed CD.
The rest is as good as self-explanatory.

The pen is mightier than the sword. But if you literally want to have some impact, use a typewriter

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