It sounds like someone put a password on the computer BIOS. If you have a desktop PC you need to go inside and either use the cmos jumper connection to reser the BIOS or just pull out the CMOS battery for about 5 minutes to clear the settings. The battery looks like a watch battery but is the size of a nickel.
If it is a notebok you need to do the same but I don't know how.
what happened was, i had gone into the setup and put in an administrator password for it to use. and i would enter that in every time it prompted me for the password. then i went back and removed that password, so now every time i boot the computer, it asks me for the same thing, and nothing i enter in seems to work
Most dell desktops have a password reset jumper. Check your manual or view the manual online for its location. Clearing the bios will not clear the bios password as far as I know. Good Luck.