Put very simply, you can't.
No third party program you just installed can 'know' for sure what files are needed, which ones are not, and what are true duplicates.
That's why you're always asked whether you're certain you want to delete them, and if you don't know for sure, you should answer 'no' to that as well.
You can delete the entire content of your Windows\Temp folder, and most other things with a *.tmp, *.bak, *.old, and *.prv extension are are safe to delete as well.
I've found this a good way of going about uninstalling:
1) Shut down the app you're going to uninstall
2) Uninstall it in Control Panel/Software add-remove
3) Reboot if you're asked to
4) Remove leftovers by going to Start/Find/Files or Folders and searching by keyword.
Often the program's folder in Program Files and entries in Start menu remain, and can be removed manually.
If you're really finicky you can go into the registry and search by keyword there as well.
You tend to find orphaned entries in HKEY_LOCAL_MACHINE\Software and HKEY_CURRENT_USER\Software\Microsoft\Windows\Curre ntVersion\Explorer\MenuOrder\Start Menu\&Programs, among other places.
And assorted file types in HKEY_CLASSES_ROOT only associated with this program.
Don't forget to look for the name of the program and the one of its manufacturer
You can remove those as well.
(Registry Tool tab) and <a target="_blank" href=http://download.cnet.com/downloads/0-10106-100-881470.html?tag=st.dl.10001-103-1.lst-7-1.881470>RegClean (http://www.vtoy.fi/jv16/shtml/powertools.shtml>JV16) certainly do a decent job as well.
It would be even better to try and stop all this junk from accumulating.
In my experience most uninstallers leave an enormous amount of junk behind.
Here's an interesting article in that respect: <a target="_blank" href=http://www.woram.com/CLEANUP.HTM>Registry Cleanup Techniques</a>
To avoid this in the future, use an install monitor like Ashampoo Uninstaller (http://www.zdnet.com/downloads/stories/info/0,,77424,.html>InCtrl5</a>
It) a lot.
This is a kind of a cross between an install monitor and an uninstaller.
Warmly recommended.
Cheers, Tony