Can you trust free mmorpgs

It depends. If the MMORPG in question is completely online based in most cases it can be trusted since you are not loading anything onto your actual hard drive.

If the game has to be installed onto your computer, you need to be careful. You never know when the game may be hiding something. If it is a more well known game, (World of Warcraft) you may not need to worry. But if it is a game that isn't as known you need to use your own judgment when downloading.