EDIT ON 09/25/2009 ... the nag message has been long gone, so apparently AVG decided that this was not such a good idea after all. Thanks to AVG for correcting this situation.
I've been using AVG Free for quite some time now, but a recent update has added a nag window that pops up from time to time that encourages me to "upgrade" to the paid version.
This is not good. Either it's free, or it isn't.
I Google'd about to find out if there was a way to turn this off; apparently there isn't. Except of course to buy the product. I did find lots of complaints about the nag window; a few suggested nod32 as an alternative but I've never tried it, and none of my personal contacts have either, so I have no first-hand info about that.
I had originally implemented AVG as my anti-virus solution to replace Norton AV on multiple PC's which was getting expensive, not to mention that the memory & CPU usage was pretty high. But now that AVG has added Windows services for link scanning etc. it has a large footprint too.
Now, from the vendor's perspective, supporting zillions of unpaid copies with updates for virus definitions can be a heavy load. But IMHO if the product was labeled as a "trial copy" then the nag would be more acceptable to the consumer - take WinZip for example, which reminds you that your copy is unregistered every time you launch it.
Recently, broadband providers such as Cox have started offering free AV software to their subscribers; apparently preventing virus outbreaks & keeping their network healthy outweighs their cost of the software; so products like AVG, free or otherwise, might someday cease to exist.