You can use the install DVD to reset the password, repair the disk and repair permissions. If you don't have the install DVD you could try the following.
Print the following and then follow these instructions very carefully - at each stage wait for processes to complete - spaces are important:
1 - shutdown
2 - start in single user mode by holding command (Apple key) and S while it starts
3 - type the following and hit the return key to repair the disk
/sbin/fsck -fy
4 - type the following and hit the return key
/sbin/mount -uw /
5 - type the following and hit the return key
rm /var/db/.AppleSetupDone
5 - type the following and hit the return key
shutdown -h now
6 - start the Mac
7 - The above tricks the Mac into thinking it is starting for the first time. Follow the various processes as normal but DO NOT use a user-name that has been used on the Mac before - if you do it will erase the original account. It will set up a new admin account without removing existing accounts with different names.
8 - Use the new admin account
9 - If necessary enable the root user (see Mac help) to reset passwords on other accounts.