Hi, these Vostro 3500 models unlock with the 7-character Service Tag and with the 23-character PPID code that start with CN0 and ends with A00 (ex.: CN0....A00), with these a master password is generated and it will help you reset the Setup and/or System Passwords.
The ServiceTag and PPID code are usually shown on screen, but if they are not, then it's taken from bottom cover (ServiceTag) and the PPID will be inside the battery compartment area.
I've unlocked a few of them, my suggestion is to contact Dell and get a service quote, then you can go to eBay and find the solution there for a reduced price (compared to Dell).
Regards,
Alex