I have 2 Problems:

Problem 1:
I have a file in my c:\oracle\ora81\network\log directory named nmi.log. This file is almost 2 gig in size and is a text document. What is this and is it really needed.

Can I delete it to free up some disc space .. ?
How can I control the size of this in the future?

Problem 2:
In the same directory (\oracle\ora81\network\log ) there is this file SQLNet.LOG with this error message: TNS-12640

What i have to do resolve this error ?

Thank you