Now even I'm confused ! the Windows XP Splash screen/Progress bar indicates the system posted, and yet the images are of a BIOS password prompts which will appear before Windows is ever given control. OK, I see from the new images that it is the Dell BIOS startup, not the Windows XP Splash screen, now I can wrap my head around it a bit better.
I'm starting to wonder if perhaps the passwords are, for some reason, stored separately and not "battery-backed"? In which case clearing the CMOS would do nothing. The Manual has an appendix about 'forgotten' BIOS passwords- I think it's safe to say you've not forgotten it but not being able to type it is as good a reason. According to it (Page 86 of the aforementioned manual) you should be able to reset the password by moving a jumper on the motherboard. The Manual shows a image that should help you find it- it is labelled "PSWD" on the motherboard. Effectively you move the jumper from the two pins it is on of the three to the "other side" of the three pins. (rather than connecting to pins 1 and 2 it will connect pins 2 and 3). if they are in fact stored separately- which appears to be the case, given it still prompts for the password, you may be able to clear the password using this method to allow it to continue to boot.
One other thought is whether anything might be causing the keys to stick down? "Keyboard failure" will be shown for stuck keys or keys that are pressed at power-up, for example.