For its time it was a good spec with Vista 64 and 4GB RAM and should still work well today. Three options.
1. Try logging into the administrator account in safe mode.
Start your PC and press F8 repeatedly before Windows loads to open Vista's Boot Options screen.
Select Safe Mode and press Enter.
On welcome screen choose the Administrator account, which by default has no password. Just hit return.
Goto Start / Control Panel / User Accounts and create a new user account with admin privileges
Close and restart PC and log into the new account.
2. Use a password recovery program
The following link shows 8 free password recovery programs.
3. As a last resort reset to factory defaults. Dells normally come with a hidden recovery partition.
Either boot from the recovery disks you created or use the hidden partition for a fresh install.
To do a factory restore,on startup press Ctrl + F11 which will take you to the recovery partition so you can do a factory restore. This will then create new user accounts.