I used the DeusEXE mod because Steam was screwing with my copy, and I have a problem. For some reason, I can't go full screen. Can anyone please help me?

Under the Configuration tab (I assume you mean you're using Kentie's launcher) you have to check a box for fullscreen.

I checked the box, but it still stayed in windowed mode, detailed textures are also disabled permanently despite that check box being checked.

Are you using Windows Vista or 7, and if so, did you install to Program Files? Vista/7's User Account Control may be preventing the launcher from saving its' settings.

I don't think it should do that either, since (I believe) kentie's Deus Ex launcher saves all its data to your Documents folder, even the Deus Ex .ini files, so UAC shouldn't be altering that. Definitely strange. Check your documents folder to see if there's a Deus Ex folder in there with those files, at least.