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 M