Safe?

Not in Canada or the USA. In China it is, but that is because the government is incredibly corrupt.

Hacking a game is modifying memory in your own computer which you have every right to do. It is when you distribute cracks, etc... where it can become illegal because cracks in particular violate DMCA (they contain a lot of content that the 'cracker' does not have legal licenses to distribute.) If the original executable image is patched at run-time to detour the cd\key checks, then it is legal. Distributing 'hacks' is usually legaly too because they are soley written by the reverse engineering modifying the game. Any patches or modifications to the game are authored by the reverse engineer. However, if hacks contain large enough signature (for signature scanners) they can become illegal (for distributing something they don't have the rights to.)

Exploitation is a bit in the gray area.

Also, OT: I've never gotten a virus from tpb, just be careful; choose something with a lot of seeds and legit trackers.