There is no guaranteed, surefire way. I'd suggest trying to get the
installation media somehow or other - what was it that wasn't provided with
a method of reinstallation? Is the program legitimately yours?
When I've been in this situation, this can sometimes work - say for example,
the program is "Really Useful App" by "RinkyDink Solutions, Inc."
1) Via thumb drive, network, crossover cable, CD etc copy over the
application's folder, eg C:\Program Files\RUA, from computer A to computer
B. Note where the shortcut(s) on A point to, copy or reproduce them on B,
see if the program works - it just might. (Don't forget to enable viewing of
hidden and of system files.)
2) If doing that throws up any error messages about missing files, locate
them on A and copy to B. If files are DLLs, they might need registering.
3) Search through the registry on A for any entries with names remotely
resembling anything to do with "Really Useful App", starting with the most
distinctive - eg "really useful app", "really", "useful" (but not "app" or
you'll get more results than you can shake a very large stick at), "rua",
"rinkydink", maybe "solutions". Reproduce these entries on B and try again.
In your registry search, tick searching for Keys, Value and Data, but not
"Match whole string only". The usual registry work caveats apply (care,
backups). If you're not happy footling around in the registry then don't!
4) Search A for any other files that might need copying - eg .ini files,
particularly in C:\Windows and its subfolders. Maybe run the program, make a
small change to its options/settings, then search for recently-changed .ini
files, eg rua.ini. (Then reverse the change.) I'd recommend using Agent
Ransack for searching rather than XP's built-in module as the latter needs
tweaking to make its results (or lack of) reliable:
http://www.mythicsoft.com/agentransack. Copy any such files, test again.
As I say, there's no guarantee; there could well be obscurely-named or
encrypted registry entries that you've no hope of finding (eg related to
licensing). Well, you *could* try running the app with a registry-monitoring
tool... If you *do* get it working, then on your archival CD you'll need all
the files you copied as well as a note of the registry entries you made.
That's what I'd do - good luck!