The best way to prevent the infection of course is to get the user to stop downloading.... that is what Phantom010 is pointing to in reply about our Rules.
We cannot help you with how to use P2P apps--- but your main question is not about using P2P but about how to have an easier time of it....something that tons of people using this site have been allowed to do and gotten help for it. Might not be right out in their questions, but in logs we often see P2P apps and still help with an infection, and along with that goes pointing to prevention
You should be allowed to discuss preventing malware infections, so I will address that part of your question. Though it might not keep the user from using P2P it may just educate her a little.
There are two lists here
http://en.wikipedia.org/wiki/Real-ti...ime_protection
Read the stuff and check the links
The pay-for types might be more robust and have the needed real time protection, but I use free ones and they seem to work very well.
I don't download that way, though so I cannot say if she clicks on a cracked download or a keygen type thing....whether it would pick up spyware in the download, that is a matter of trial and error for you and the customer to go through. Does seem that a good program would indeed alert the customer, but at first they just do not get it and will shut the program off etc to get the precious downloads....
That's one reason some of us refuse to provide assistance when we see that most likely, the root cause of a problem is P2P....however we are not automatically prevented from assisting in malware removal , nor malware prevention.