Given that this is Superuser, I'd suggest the following definitions (closer to the original meaning of hack):
Hack: Whacking/twiddling some system internals in a way not easily accomplished via standard GUI functions, usually accompanied by a disclaimer that the solution may break the computer if not done with care
Malware / Security: tags to use if you think you've been hacked, or if malware has messed up your system and you need help fixing/recovering it
We don't really need hack to be related to malware or security, since we have those tags. And if someone asks how to hack into a bank server I imagine we won't be assisting. So those meanings of hack don't need to be covered by the tag.
The tag 'hacked' in past tense could be used for people suffering security / malware issues if we really need another tag for that. This would differentiate from 'hack' in the sense of messing with the registry etc.