Tifur, I had the same problem with another trainer. Open up explorer and check out your root directory, should be C:\ if it's default. Go into folder options and under the view tab you need to make sure you have BOTH show hidden files checked, and hide system files UNCHECKED (in 7, maybe vista too).
Now go and see if any hidden files show up in your root folder. Specifically any that are .dll. There is NO legitimate reason for a .dll to be in your root directory. The one I saw that caused me problems was SXC.dll or something like that. I've also see h4x0r.dll too.
Double check that hide system files option, it's what was doing it for me.
Well i checked all , there are no .dll´s in any of my root´s on all 3 Harddisks. All fine. All other Trainers from here work fine ( Mass Effekt 2 , Dragon Age etc ) . Only the AvP dosnt want. But i found a way with Art Money to keep my Ammo on max , this is a bit ugly because i have to search the Values on evry Mission new , but better then nothing. With enough Firepower my Soldier should stay alive ^^