Well, just because it doesn't kill your disk, this doesn't mean a worm, virus or otherwise malicious program won't cause harm to others. Even if you're very very careful it's almost impossible to ensure your computer doesn't turn into a spam relay or tries to infect other machines, for example, by exploiting bugs in their operating systems or applications.

There's exactly one safe way to use untrusted software: Put it on a CD or DVD, insert the the disc to a computer that is never, ever connected to any network - neither local nor the internet.

That's paranoid but it's the truth nonetheless.