There's a really old simple local exploit you can use to create accounts, change priveleges to accounts, and even passwords. Even with the release of XP they still didn't fix this local exploit, it's still not patched to this day.

All you do is reboot into safe-mode, open MS-Dos and type in "net localgroup administrators /add limited". This command will give administrative priveleges to any limited privileged users using the computer, including guest accounts.