>It's installed in c:\Program Files\Text2PDF. In my program I just check, if this file is present there and if not, display a message to download that program. However, what if a user installed it to other location?

If it doesn't create registry entries that point to its location, than you've only two choices:
1. Search all local HDs
2. Ask user about location

In either case you'll have to remember it's location so you wouldn't ask about it in the future.