I'm ugrading my work computer. This is like a 50% power boost so it's worth some time. I put more time in it than I had to; this is a single-day affair if you're being slow and doing regular work in between telling the computer what to do. It went wrong in various ways, but finally I was able to transfer Windows XP from one computer to a totally different machine, with all my programs and files intact. When I finally found the proper method, it was super easy! NO extra software is required if you have two legit Windows licenses and a Windows installation CD.
First I used the Seagate-branded Acronis disk copying software to clone my hard drive in case something stupid happened (and it did, repeatedly, because I didn't do things in quite the right order the first go-round). Then I set the original disk aside and booted the clone hard drive, told XP to upgrade itself to the version of itself on the XP installation CD, then shut down the computer. Instead of letting it reboot and "complete" the update on my old box, I moved the hard drive to the new machine and let it continue with the "upgrade".
I had seen the MichaelStevensTech page with similar instructions but it was presented in an intimidating manner. Then I found this page at Development Stuff that covered it in more-accessible language.
Paul's Computer Service saved my bacon when it all seemed to work but hung at a black XP logo splash screen saying "please wait". It wanted a newer version of Internet Explorer than was on my XP installation CD. Boot into safe mode, install IE, install chipset drivers, reboot, activate, download a couple hundred megabytes of updates and we're done!
It works like a champ now. Thanks everybody!
Travails along the way:
First, I tried the dead-easiest way: (clone the HDD and) slap the hard drive into the new PC. Believe it or not, this HAS worked for me in the past when the PCs were close-enough to the same configuration. This looked initially encouraging, but it ended up in an endless reboot cycle, only getting as far as the splash screen before restarting. Trying the whole thing again with a Repair Installation of XP also looked promising, but also ended in an endless reboot loop. In safe mode it would get as far as a 7B stop error (STOP! I can't read the hard drives, hoss!). I tried this a couple times and killed the boot sector on the disk somehow. I changed the BIOS SATA setting to Legacy (ATA also worked) and got past the 7B stop to an Error Loading Operating System error. That was because it couldn't boot from the disk with a broken boot sector. The data was all there, but this required a full low-level format (fixmbr, fixboot, fdisk/mbr didn't work). I installed a fresh copy of Windows on the new drive to be sure it would boot and re-cloned the drive. THEN I did it the right way.