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Boot from a LiveCD, then mount the / harddrive partition. You can now chroot into your harddrive and just run passwd as root there changing the root pw. That's also the way to change to grub password.
Presently i'm using fedora, Is it possible to use ubuntu live CD for change password ? – KumarSep 20 '10 at 9:58
3
Yeah, it doesn't really matter what distribution you use. – tanteSep 20 '10 at 10:11
@Kumar P: But you should use an 32bit CD for an 32bit system and 64bit CD for 64bit system – krissiSep 20 '10 at 10:26
@krissi: Techincally you need 64bit kernel for 64bit system and any kernel for 32bit system. So SysResCD booted with 64-bit kernel (it has only 32-bit userland) can chroot both into 32 and 64 bit systems. – Maciej PiechotkaSep 20 '10 at 13:06
@Maciej Piechotka: Thats right. But I ran into some trouble in the past with mixing 64bit CD and 32bit. Don't ask my why this happend (maybe some tools read the wrong data from /sys or /proc). Maybe it has changed meanwhile, but I keep my pendrive with both, 32bit and 64bit Linux ;) – krissiSep 20 '10 at 13:18