I prefer downloading programs bundled with cracks/keygens, because these cracks are more likely to work with the program, and because many of the files labeled "crack" you find here are trojans.
There is no way you can be 100% sure that a file doesn't contain viruses, but keeping your antivirus software up to date should help a lot.
I don't use p2p to download cracks/keygens, I download them from websites such as
www.cracks.am. But even cracks on here can be viruses, so be careful with these too.