I don't think you would be able to re-install without the disk or using the System Restore but I wouldn't worry about it. There are just as good, if not better A/V programs available for free as you seem to know. I've been on AVG for a good long time now after being messed around with by McAfee. I've never regretted being with AVG and it's certainly been less hassle tha McAfee ever was.
Go for it - I doubt you'll regret it.