Removing software

[mofo]

I'm curious as to what files remain on my system after i remove a program from Windows XP. Reason i ask this is, i installed a software "tryout" version good for 30 days only. Then i un-installed it from my system and cleaned the registry. Now i'm trying to reinstall it and it "knows" the trial period is over. This post is not about trying to break the law or pirating software, but i would like to know why/how this is happening. Can't we cleanly remove any trace of old programs on our systems and get rid of them for good?

[Statica]

The short version of the answer is you never know. When you give a software permission to install on your system, you give it access to put in as many files as it wishes in as many locations, or as many registry keys as you would like. Unlike, with other OS' where one rarely if ever downloads the binaries, Windows based software doesnt necessarily need to report what file goes where.
The same issue is true of uninstalling a software, since we do not have a list of the files or registry keys in the first place, we cannot really track them all down. All one can probably do is check for probable files based on file name or based on intelligent searches to the registry. But as you can imagine, that could be like searching for a needle in ye olde haystack.
The other option is to run some sort of registry. file monitoring software like http://www.evansprogramming.com/tracker.asp (hint: hey wait, isnt System Restore on XP the same thing )

Now to the disclaimer notice that I hope you will read: A shareware/trialware program is a wonderful way to distribute programs, it allows you unimpeded access to the program to EVALUATE whether it is useful enough for you. If you are trying to install it after the trial period, then the program is obviously useful to you. At this point, it would be a much better option to support the program and the developer by buying the software.