It might not be your computer, it could be the settings on your phone. Your phone deletes those that are older than two months and when you sync, your computer takes those changes too. Go into your calendar on your Blackberry and click your menu button and choose options, then general options. Change your "Keep Appointments" to how ever long you want, which includes forever.
I've had that happen too a few times even though I had all my settings on my bb and pc set correctly. I find that when I click on "resync" after the first sync, it does not delete any appointments and that seems to be working for now.