You're not going to believe this, but there's a fairly good phone dialer/caller/answerer that comes with Windows XP!
Start -> Accessories -> Communication -> Phone Dialer
It even handles Internet phone, and conference calls (phone or voice server), too!
As long as you have it running it will pop-up and ask if you want to answer the phone (on your machine) with a popup.
Note: It might be an option to install this application, and it might only come with XP Pro - I don't know for sure.
Lemme know if you don't have it /can't find it and I'll dig further and get it for you, k?
*EDIT*
This application is included in ALL instance of Windows XP. See my post a bit later with details on how to "activate" it....