If you're using IIS, then it's most likely the local IIS worker account on the server. If the server was named Foo, then the account name is likely IUSR_Foo.
This can be changed, however, inside the IIS Admin console. I don't have access to an IIS server at the moment and can not give you the exact location, but unless it has been changed you should be able to use the account I mentioned above in the folder's ACL to give PHP the proper permissions.