my current project sells software online, and i would like to track the sales, where they came from and which sales channels perform best etc.

My issue is that the user can download the software and use a trial for 30 (sometimes up to 60) days before the software reminds him that the trial is about to expire. This means each successful sale would typically have 2 "visitors" in GA, one when the user first arrived (lets say from adwords for "Awesome software trial") and the next visit when the purchase happens.
How would i go on about tracking these sales in google analytics, and essentially link them back to the previous session when they first arrived to download the trial? Anyone with a good tutorial on this topic?