To continue Jig's point, there is supposed to be summat like 170 years between Archers' encounter with the Borg, and 'First Contact'.
The more time that passes between an event and the 'present', the more likely that facts will be forgotten. (Or misinterpreted)
As time passes, things also change. As Jig said, records wouldn't have been as good as the ones in TNG times. You just have to think about things today - we can record information in writing, in sound, in video, on a computer, and so on - 170 years ago, such things were impossible!
Try to imagine what technology we might have in 170 years time (If we survive that long 8O) - we can only try to guess at it, coz we don't have any idea! If we did, what would be the point in research? We'd already know everything! 8O
Over time, things get lost or forgotten, the way information is recorded improves, and we record more of it.
The simple fact of the matter is that no-one was around to remember it, (Were the Vulcans informed of the Borg?) the information could easily have been lost, and if any info did survive, it would likely have been lacking indetail, compared to the standards/needs of TNG times.
There was one thing that people seem to have forgotten about the encounter though - the subspace message. Even if Q hadn't sent the Enterprise to meet the Borg, they would have found their way to the Federation eventually.
So even if the paradox was solved, (Obviously it can't be though) the Borg were still gonna attack, only at a later time.
