How do demo progs know the exp date?

I have always wanted to know how time limited demo programs know when the program has expired. I have switched to back up registry files, scoured the computer for any remnants of the program, etc, but somehow it *knows* i had previously installed it, lol

Does anyone know the trick to faking out the demo prog? There must be a litle tickler file it looks for that tells the first install date, with some weird random name, and some phoney create date (making it very hard to find). But that is only a guess.

Is this a subversive post?

good question, I assume only those who actually develop such demo programs will have real answer... but on the other hand they're obliged to their company and revealing such information won't do any good to the company so...
anyway, my guess is that Windows itself keep detailed log of all the programs ever installed. I know for sure XP is doing it, dunno about older version. Moreoever, when I delete such log file (as it had virus) it come back on the next boot - so Windows is keeping such log, and backup and probably more backup. invincible.
so, the program just hook to this log somehow and grab the relevant information.

another option is that it use the registry to store the date, but it's using key name that has nothing to do with the program name itself, so you can't find it in any search. it can use for example the classid of the program - that long weird number.

bottom line, I fear that until someone who actually deal with such programs will pop up and read this, you'll have to pay for the full version, or keep trying.....