Microsoft Licencing

I maintain, troubleshoot windows systems. If a customer brings a system to me with a problem am I legally bound to veiw his/her microsoft license? If they cannot provide me with a licence, can I legally work on the pc? If windows needs to be re-installed can I do this from my original disk and insert the customers licence key? Can I pull this key from their registry if they do not have a hard copy?
I am finding it difficult to obtain direct answers from microsoft regarding ‘real world’ scenarios.
Your help would be much appreciated, both in direct answers and/or links to further information or veiws.