thanks barhumbug,
after much frustration i succeded to recover my xp system and also a lost partition. the problem was caused by deleting the ubuntu partition which was a second operating system but always was the default sys at startup.
many and repeatedly trials to get the BIOS screen at power-on failed, only displaying: "GRUB RECOVER:".
not knowing what to do i accidentally, one day, persistently hit DELETE key, right after power-on, for a few seconds and ...WOW... BIOS screen appeared !
from here:
- boot with xp cd
-selected R (repair) when prompted
- typed the commands
fixboot
fixmbr
and after exit windows xp was loaded properly
(i had to be administrator)