Your are right you don't know what you will get if you search for moon.tga.
But afaIk there is no way to crack the hash calculated from the file (put a corrupted file on the Net with the same hash).
If you have eMule and click on my moon.tga-ed2k-Link you will get chunk for chunk exactly my moon.tga checked for corectness (if there is a virus in it, my f***** wintendo is to blame). Even if my "original" get altered (virus etc.) by clicking on the link above you will download the original from other sources (I the file was spreed throu the Net).
So if we use ed2k-Links just one can corrupt the file (the poster it self) and he has to do it before he hashes the file (tell eMule and co. to make a link).
The question is not can I trust P2P, it is can I trust this Forum
PS:
1. If you plan to check P2P out make sure there is an updated (once per day) antivirusprogram an your wintendo, there is no real need for uncommon Operatingsystems as 99% of the virus are writen for windows but I still recomed it.
2. Do NOT share your hole Computer (make a folder put all you want to share into it and share the folder and subfolder if needed)
3. Trust NO file you searched by NAME but if you can trust your friend and he put a file in to the P2P-Network plus gave you the ed2k-Link you CAN trust the file it will download. So if you want a file do not search for it by name insted get the hash an search for it by hash.
Happy Downloading