It could be a registry key or even just a file stored somewhere on your computer, to be honest though I hate these methods of 30 day trial software, because if you do trial a lot of software, you could end up with lots of leftovers that don't get cleaned up and just increase to the already large registry and waste space on your hard drive.
I mean the easiest way to understand what a program is doing is to monitor it, so you need registry and file monitor tools. When you begin installation of the program, have these monitoring tools monitor just that program and see what it does with installation, save that monitored information, then monitor the program when you first run it, usually this will most likely always check the evaluation period, so it may pick up the files/keys that are part of the evaluation period. Save this information too.
The installation, may or may not have checked the evaluation period, but if installation never went through because of an expired evaluation, it most likely had to check something on your computer for this, and if you were monitoring it, you would have picked it up. You could keep running the monitoring tools on every installation, until you determine all the things that are similar with each installation.
The thing with monitoring tools however is knowing which file/registry entry is the right one.
What I suggest is just a manual clean up of the registry of that program.
If you know the Vendor of that Program e.g. (Mozilla, Microsoft, Nullsoft, etc). You could remove the keys from there that refer to just your program but before doing this, always back up your registry and make sure you uninstalled the program.
So open up the Registry Editor by going Start | Run and typing 'regedit' without quotes. That will open up the registry editor, which is like a Windows Explorer interface with Folder Panel.
The locations you're interested in should be
HKCU\Software and
HKLM\SOFTWARE and you need to find the vendor of your software, go into it, then see if you can see the name of your program within it. If you've backed up the registry, then click on the name of your program folder in there and delete it, make sure you check both locations for HK locations for it, and do the same. Now this does not guarantee it removed the evaluation period, this could be stored deeper inside the registry, which only the tools could help you understand this or you knew how to disassemble a program and understand what it does, but if you knew how to do this, you wouldn't need to ask this question at all.
There's probably other things I've missed out on how they do evaluation periods, but it could be simple to something very complex. Sometimes just reverting the calendar back can work for you.
Cheers,
MC
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