IVM is a telephone answering machine, voice mail, call attendant, and interactive voice response (IVR) program for Windows. It connects to the phone using a standard voice modem, professional telephony board or even directly to a VoIP service.
This software is an effective voicemail, call attendant, info-line, audiotext or autodial solution for small to medium businesses. For example, IVM can redirect in-coming calls during office hours or act as a PC answer machine and take messages for a number of voice mail boxes after hours. All calls (including those answered by you) are logged with date, time and caller ID. The recorded messages can be played at any time, forwarded to an email address, accessed via the internet or, if necessary, saved for future reference.

IVM can redirect in-coming calls during office hours or take messages for a number of voice mail boxes after hours. All calls (including those answered by you) are logged with date, time and caller ID. The recorded messages can be played at any time, forwarded to an email address, accessed via the internet or, if necessary, saved for future reference.

IVM can also be used very simply as a computer answering machine. Just run the install file on your PC and it will be ready to answer calls. But, when you are ready, you can explore the advanced key response and programming options to create a sophisticated telephone interactive response system. You will find many exciting ways to use IVM to provide information, serve your customers and save time.

http://www.nch.com.au/ivm/

Tight.  Runs as system service with control interface.  Earlier versions were a bit sketchy, not properly handling terminal services logins on the box running the answering machine.  But version 3.0 onwards were great for me.  Emails you MP3 attachment.  Works great with decent hardware.