Well, now that I think about it, it's a lot to build an install, and maybe harder if starting from now. Search and read about nlite, nlaom, makecab, OnePiece, RVM, and a little blackwingcat for study. M$ has utilities to manually do the same thing, cdimage.exe might be hard to find. I've made many legit disk. The last I registered was last Nov I think, I committed a retail MediaEdition to it, my last ever, a dual xeon 5672 with 4GB AMD GPU and 24GB that runs ~almost~ passive cooling.. Very close to the top of what's possible with XP 32, yes it uses all the ram and vram. I retired the Q35/6600 months prior, the third on that license. Frankly, a whole bunch of knowledge that's not that useful anymore.
A note on the 30 days, as stated you can reset that, google rundll32.exe sysetup something.
If you reset right before you shut it down, then image it, it will boot up whenever with 30 days left.
You close enough to just polish what you have and make the reset image.
And YES, you are allowed to remaster a RETAIL CD. Some computers came with OEM copies tagged to the bios, some came with RETAIL licenses. Actually, the license allows you to migrate/upgrade just not install on multiple computers, which kinda includes using the disk for endless grace periods. An image for diagnostics? That's fair I think. Make a qemu-kvm based image and you might as well give it a retail registration, I think you still can. Since I took the pill to make me forget all this at the EOL in 2014 or whenever that was...I've (re)built a few dozen special purpose XP's, they all have registered fine. Actually one did fail, not my issue, they had another box to work with. Forcing obsolete things to a value of ZERO, not fair...The 2 VM images I made years ago are registered and still work fine.