How old?

Will it take a SATA SSD?

You can get a new SSD of around 240GB for about $50.

Crucial-BX500-240GB-2.5-inch-SSD-SATA

If it ran any version of windows previously, it would fly with an SSD installed.

Even if you go the linux path, I would seriously suggest looking at an SSD if you can.

Yes if you can put in a SSD it will fee like new .  If you can add more RAM that will help as well.. 

ChromeOS.

https://www.neverware.com/freedownload#intro-text

Okay, it's an old Dell. I can't change the BIOS menu as it's locked. Google won't show me and clues on how to get the password or what the default password could be.

I have been using  Linux mint for years. I would never go back to using windows again.

As others have recommended Linux Mint is a good one to try. Its a terrific first Linux distribution to try.

Personally I find its not for me and prefer Ubuntu (which Linux Mint is based off). I'm currently typing this on a an old HP Probook 6460b. Its got an old i5-2410M with just 4GB of RAM running Ubuntu 21.04. Terrible screen on it mind you :D  Aside from the screen, the experience is great. Can run all I need and is perfectly fine for "office" type applications: web browsing, email, spreadsheets and documents. And its snappy to do all those things.  I mention this as you haven't really specified what specs you have available.

I would also second the investment of a SSD if you can. That makes a massive difference to load times.

To simply try out Linux,

• Knoppix is about as basic & straight forward as it gets
• I personally like Pop!_OS from System76. It feels less bloated than Ubuntu & Mint.

If its really old or low powered Lubuntu would be a good place to look or maybe even Puppy Linux, depending on specs.

The question thats needs to be asked is

What are the specs
How much RAM . What CPU .
Some old laptops are limited to 2Gb of RAM MAXIMUM , so that could be a limiting factor even if you buy an SSD .
If it only has 1Gb of RAM, then something like PUPPY (is it still around ?)

A clone of ChromeOS is another option , if its compatible with your old laptop
https://www.neverware.com/freedownload#intro-text

Whether a Linux OS is heavy or light isn't really decided by the distribution it's decided by the desktop environment. Linux Mint's flagship version is the cinnamon desktop environment (DE), it's one of the heavier DEs (still much lighter than Windows 10). The lightest (while still user friendly/no terminal use necessary) DEs are LXDE, XFCE and LXQT. I'd recommend Lubuntu (LXQT) or LXDE Fedora for best possible performance. 

Don't know if there's a best, but you have quite a few to choose from that are lightweight. Examples are Q4OS, Linux Lite, Ubuntu Mate, Zorin OS Lite, Xubuntu, Lubuntu, Linux Mint XFCE, Peppermint, Bodhi, Puppy and more. 

Just-in-time …

Definitely try a couple light Linux distributions before you pay for a SSD or more RAM. They are massively lighter than Windows 10 and even Windows 7, 8, 8.1. You may not need any upgrades.