How do I reset the password on my old Dell Dimension XPS D266, Without opening the computer itself?

Since this PC is really old, chances are the motherboard battery died long ago. So it would have to be replaced before you can do anything else...

Meaning you have to open the case, or just put it back in the attic.

@RoHe​  I have opened the case, (phew that was hard) and surprisingly, the battery’s alive! I hooked it up to one of my monitors and yes, it boots! The only error is that the date and time setting are off, which is obvious.. and I cannot remember the password for the life of me (still).

Amazing the motherboard battery still works...

Windows 95...? 

What happens if you just hit Enter without entering anything in the password box? 

If that doesn't help, look in BIOS, for an option to clear the OS password.  Or you could try clearing BIOS:

1. Power off and unplug power cord from rear of PC
2. Press/hold PC's power button for ~15 sec
3. Open case and remove motherboard battery
4. Press/hold power button for ~30 sec
5. Reinstall battery (might be good time to install fresh one, even if old one seems to work)
6. Close up and try to boot now.

@RoHe​ Thanks, man! 

I will try N' do this later on, just to be sure can you show me an image where the motherboard battery is? just so I don't unplug any thing important, haha! 

@RoHe​ Pressing enter in the password box does not work, sadly.. By the way.

Too bad pressing Enter alone didn't help...

This PC is so old Dell no longer has any of its docs online at their support site. The support page does say this:

For the best possible service, please provide the name of the product and your preferred language to manuals_application@dell.com and we will email you the document if it's available.

It should be a standard CR2032 3-volt coin cell battery (about the size of US quarter) and easy to spot.  There's an image of the motherboard here, and the battery is to the right of the 4 white slots.

You may need a non-metallic "tool" to release the retention clip(s) to free the battery, if using your fingernail doesn't work.  Just remember to note which side is up and install new battery in same orientation. And be careful to insert it under the retention clip(s).

You might search the web to see if somebody has a way to reset Windows password in whatever version of  Windows this PC has....

@RoHe​ I took the battery out, N' swapped it and it still requires a passcode.. guess I got to figure out another way..

Did you press/hold power button for ~30 sec after removing the battery, before reinstalling it?

@RoHe​ Yeah, I did hold it down for around 30 seconds.. I shall try again, though. Wish me luck! haha!

@RoHe​ It did not work, and going into bios also requires a password.. 

yikes.. this may take a little while..

@RoHe​ Ah, well it was worth a try, thank you for your help! 
I might just buy another mother board so I can still use the cds I have lying around for it..

Thank you!

You may not need a new motherboard. 

If you can boot from either a bootable USB stick or from a Windows installation CD, you may (should) be able to wipe/reformat the boot drive and do a clean install without needing a password to do that or to log into Windows afterwards.