Questions:
What Operating System are you running?
Do you have the installation disk for that OS? (Not a recovery CD that the Dell or HP ship with their systems)
Do you recall setting a password for the Administrator?
If you've never setup a password for the Administrator, it should be blank... as in no specified password. At the login prompt (dialogue box) Just hit the "enter" key and it "should" let you in.
Username - Administrator
Password - leave blank and hit the enter key
If you have setup a password for the Administrator - and don't recall what it is - you can recover/change it if you have the full installation CD. If you don't have the install CD, you can check out
this page with basic instructions and a link to a "rescue CD" to download, burn and use to recover/change the password.
Unless your box has been hacked/cracked from the WWW or someone playing a prank on you (that had physical access to the box), it sounds like the screensaver is what's got the system locked up. That's just a SWAG though (scientific wild as* guess).
Another option that
should be doable:
When you (re)start the computer, keep pressing the F8 key. This will give you the menu to choose what mode to boot into. Select "Safe Mode" (won't need networking). Here you can get into the control panel and edit/remove passwords.