Re: Windows password problem
^^^ What he said.
Sometimes you can get lucky if the previous owner didn't set the Administrator password. Try using username "Administrator" and leave the password blank.
Once you get in, you may find that you'd rather have a clean install of Windows. I know I would.
If it's a newer system, there's a good chance it'll have a recovery partition you can boot to. From there, you can restore the laptop to factory defaults.
If there's no recovery partition, you can use the Dell recovery CDs. These are the CDs that the previous owner either lost or claimed they never had (a little tech humor there).

You can get replacement CD(s) from Dell. Dell often has separate OS, Driver, and Application CDs. You'll likely want to get the latest drivers off their web site, but it's good to have that Driver CD if you can get it.
Remember to get current on all Windows Updates and Service Packs too.
Don't forget about anti-virus software.
Once you have your new lappy tweaked out to your liking, do yourself a favor and grab a copy of Acronis True Image and backup your hard drive on it. You'll be good to go.