Computer Help and Support

If it's an old machine, the hard drive bay might not be suitable for an SSD but a regular old HD will be easy to do.

Try working on the theory that the machine is useless now, so what have you got to lose? And you may learn something useful.

There are only a few part on the bottom of the case that open: battery, RAM and hard drive compartments. Look! I found you a pic.
https://en.wikipedia.org/wiki/ThinkPad_W_series

I think the middle cover is HDD. Unscrew, carefully remove noting how the connector works, slip in a new one, reverse process and you're done. It's fiddly but not difficult.

Bottom side of a ThinkPad W520 with removable covers for modular components and replacement battery with extended capacity


On edit: I'd suggest taking off the bottom cover, locating and removing the hard drive BEFORE you purchase a new one. Then you can look at the cable and connector - I don't think this machine has an IDE cable but it might. Those are the old flat, ribbed ribbon cables. Just make sure that what you put in has all the same features as what you're taking out.