Actually there is a hard reset on the Inspiron 531s.
Make sure you have backed up all required data first. I am sorry but I do not like Dell's own Emergency backup.
1. Switch on PC
2. Let the BIOS loading bar fill up
3. Start pressing F8 (Do this a couple of times to make sure it works, overpressing is bad though)
4. Select the first option, Repair My Computer
5. Enter you password and username on the PC
6. The last option should be a Dell Factory Restore
7. Follow the wizard, and the PC will be back to the state it came out from the Factory.