But if someone installs a trial version they can read the agreement before buying it. Is that not legally binding?
Well it depends on the country, but in Denmark, clicking an "I agree" button does not constitute a legally binding agreement. Unless you can prove it, by means of a signed contract, chances are good, you'll lose.It would also depend on whether local legislation (or probably the judge) considers the trial version and the full version as being the same product. If they aren't considered the same product, you might get away with it.
Probably your best bet is to do what nVidia and Sun Microsystems do. Present you with the EULA before they present you with a download link. Still the EULA could be invalidated for any one of the other reasons.