How likely is it that there may be viruses, worms ect?

A friend got his laptop from his friend and now he has given it to me - he can't do anything with it

Now I wanted to set it up from scratch and because that means viruses, worms or, if they are specially encrypted, only deleted under a large amount, how do I best do that?

Get a bootable Linux rescue CD or a corresponding stick, change the boot sequence in the laptop BIOS so that you boot from this CD / stick and then format the laptop's hard drive including the creation of a new master boot record (MBR).

I always take out the old hard drive

and put an SSD in it.

With the opportunity you can really clean the inside of the lap.

Well and see if you can increase the RAM.

The battery can also be replaced.

Then you have an outdated device that you now also know.

It doesn't get cheaper than that.

If you delete and recreate all partitions, every pest is gone.