I finally got around to following up on this.

Yesterday, I booted my (only) WinXP box from a Linux USB stick, and created an exact bit clone of the 1TB Win-C drive to a second mechanical drive. This was after first running defrag from within WinXP.

Next, I ran gparted on the clone, and told it to shrink the Win-C drive from 1TB down to about 100GB.

Then I installed a brand new 128GB A-Data XPG SX900 SSD, and bit-cloned the newly shrunk Win-C drive to the SSD.

Finally, re-ran gparted to expand the Win-C partition on the SSD to the full space available.

Removed the USB stick and the mechanical drives, and the system booted up into WinXP from the SSD without a hiccup, apart from the mandatory filesystem check that gparted tagged it for.

So now my one and only MS-Win box runs from an SSD, and the partition is small enough (120GB) that I can save exact bit-perfect replicas of it for disaster recovery.

I'm happy!