Advice on Software Asset Management

Hi everyone – I’m new to TechRepublic, so….. If I have 20,000 PC assets spread across 15 countries how can I best perform an ongoing software audit (initially focussed on Microsft installed applications). The audit should be ongoing in that I would like to continually monitor software usage. What kind of data issues may I face? What are the best/most scalable Software Asset Management solutions available to help me. How long would such an audit normally take? Thanks for your assistance.