There's too little information to give you much of a handle. For instance, what is considered "installed software"? If I grab some sources from the web (or usenet...), compile them, and store the resulting binary somewhere below my homedirectory, is that "installed software"? Is it "installed software" if it's installed in /usr/local/bin? What if it's in /usr/local/bin, but chmodded 700? If a package installs 50 binaries, are those 50 pieces of software, or just one? But how would you find out? Perhaps you only have to consider software installed using the package manager(s) on the machine - in that case, look up the appropriate commands for the managers. Or perhaps you want to know if certain functionality is available - then you may want to look at autoconf (or something similar).

Should I use OOP?

Whether you use OOP or not isn't an interesting question at this moment. You first have to hammer out the specifications of your problem. Your last question is like I have to design a house. Please help me in designing it as efficient and clean as possible. Do I need a wheelbarrow?