try to get your hands on a program called ERD-commandor..
set your BIOS boot sequence to boot from CD-rom and start the program..
using this program you will be able to reset all passwords.. including admin passwords..
how to download ERD-commandor?
It won't help (and isn't free). You must call Dell with verification of ownership of the system. The call will cost you $60 -- it may be cheaper to simply buy another used C640 system, since the system is probably worth less than that.