G'day Kate and welcome to the forums.

I think your solution is a larger capacity hard drive with a clean install of the operating system whatever it may be as you have not told us this, nor the model, and use the current drive as an external with the OS removed for storing music etc. If the drive is say 1TB and full you need to clear it right out as that is a large capacity drive. If on the other hand it is say 200GB, that is small by today's standards.