Linux

You can get the password from the owner if you are in contact with that person. Then you can change the password or login as that person create a new account for you then delete the other accounts. Or you can reformat the HD and start from scratch.

I'm assuming this is a Windows NT/2K/XP/2003 machine. I've used this solution with success at least 3 times in the past several years. I have not tested this on XP, but worked great on NT and 2000. Here's a link to the utility. http://home.eunet.no/~pnordahl/ntpasswd/

it depends...
Do YOU know the root password...:
Yes: No problem just reset all other passwords
passwd <username> <enter> etc...

No: Is the previous owner able/willing to tell
you the password. Yes, wel just do the above
after reclaiming the root account.

If also this is no, then try to get hold on a
cd like knoppix et al. Boot from that CD,
mount the root file system, find etc/passwd
on it.
Check the line with root on it.
if it contains "root:<not a lone x>:...." then
remove the part between the first and second
: sign effectively removing the password.
if it contains "root:...." then goto the
etc/shadow file because the shadow file is used.

then edit the shadow file and remove the string
between the first and second : char, thus
removing the password.

After removing the password like this, boot the
normal system and logon as root, no password will
be asked, then ensure you set the password on the
root account and change all other accounts if
they have a password something else then
: in the /etc/password file. Then change the
password for that account
if in the /etc/shadow file there is something
else than :!!: or :*: than change the password.

"!!" or "*" as encrypted password effectively
means that you cannot logon with these accounts.
They are mostly for services provided by your
system.

Kind regards,
Nico

yes u can break it if it is grub loader then on boot option screen press " e " key of keybord to edit the boot loader setting n go to the second line of boot loader then again press " e " key n give a gap after all words and type " 1 " for single and press Enter key then press " b " key to boot into modified loader. now it will boot the system in single user mode or root without prompting any user or password then u can changer the password by " passwd " command for root. same procedure u can follow for LILO boot loader but to edit u have to press Ctrl+C first and then u can edit it by the same procedure. i hope this will work for u. best of luck