Hi Kwin, it's called a "false positive" - it's a mistake by the anti-virus checker mis-identifying the MoI trial version as a virus.

It's probably because the trial version uses a system called "WinLicense" that makes it more difficult for hackers to remove the trial time limit.

You should override it and allow it to install normally. If you need further reassurance you can put in the trial address:
http://moi3d.com/4.0/trial/moi_v4_trial_setup.exe

into the URL option at https://www.virustotal.com which checks it with something like 40 different anti-virus engines, none of which flag it as having any virus in it.

- Michael