You can avoid reinstalling everything using the "single user mode"...

the procedure varies according to the boot manager used, if you use Grub proceed as follows:
activate the command line by pressing the button "c" and define that the kernel must be loaded in a "single"

kernel /vmlinuz- single ro root=/dev/hd

and then boot the system with the command:

boot

after loading the runlevel 1, at the command prompt, simply change the password with the command:

passwd

but if the boot manager used is Lilo proceed as follows:
at boot of system, at prompt "linux:" type the following command:

linux single

even in this case after loading the runlevel 1, at prompt of commands, simply change the password with the command:

passwd