Ask HN: How to audit all permissions for people in Active Directory

I work as a developer in a .NET shop and I was asked to audit all users permissions (permissions directly granted to the user and via groups ...) in Active Directory. I tried to do it with PowerShell but that seem to be very difficult to do and after doing some research that approach is not recommended. I am stuck on this issue and I am new to Active Directory security. I need to do it on the cheap as we cannot buy commercial software. The directory is old and contains over 10000 users. I am considering writing my own .NET application but that might take at least a couple of months to write (i.e. I need to figure out how the security works in Active Directory and how to get all the permissions etc.). Any suggestions on what is the best way to approach this issue? Thank you for your responses in advance.