Thanks xilolee for your friendly answer !
You have pointed the right solution : making registry backup with the tiny freeware from ACELOGIX, "REGBAK" named "Registry Backup & Restore" in HIREN'S BOOT CD.
This allows restoring even on non bootable system !
I'll test this pleasant and speedy way...
Your post gave another solution, use of "eruNT"...
Thanks for helping, bro.
REGBAK works just fine !
With the right command line in BAT file in startup folder, you always keep copy of registry from the last start...
With WINDOWS XP SP2, the backup folder is 27 MB.