It is bit tricky to reset to factory default a Dell Inspiron 6400, Ctrl+F11 or F10 do not work as hot keys, so If you do not have restore disks you can follow the instrustions using recovery partition
here how I twist it
1. Make bootable CD which contains Norton Ghost or other disk image software
2. boot your system using bootable CD with Norton Ghost software, u can use Hiren's CD ISO image. if u dont know about hiren just google it.
2. Once you boot up , open Norton Ghost click LOCAL-> PARTITION->FROM IMAGE
3. now "look in" for "[DellRestore]FAT drive" and select this drive (this is recovery partition)
4. now go to "IMG" folder and select FI.GHO file
5. Click Open and there you go... process started