There are some free tools that will do this, I had to do the same thing yesterday (incidentally also for a Sharepoint related issue) and a quick Google search revealed a number of options. Personally I don't have any experience with any of them or the developers that make them so I'm not going to make any recommendations here.
What I ended up doing was using Powershell to audit the permissions. You don't mention which version of Windows you're using so I don't know whether you have it installed automatically or not (if not you can download and install it). But I found a script here for auditing permissions across multiple servers, which I adapted for use on just one.
If you want to write your own script to accomplish this (Powershell is easy, it doesn't take much to write very basic scripts), you can read up on the Get-ACL cmdlet here