Not all programs will show up in the "Add/Remove" programs list.
However, BelArc Adviser (free download; Google for sites) will give you a complete inventory of programs installed on a single PC.
For a network environment, BelManage is a good (commercial) choice.
A couple of sample questions that BelManage can answer:
Which machines have application X?
Which machines do NOT have SP2?
I am not affiliated with the Bel folk; I did evaluate their software.