Anyone interested in setting up a PC based PBX or IVR system should already be familiar with Asterisk, it is a free open sourced PBX software that runs on linux.
However if you are a noob at linux, or find configuring an Asterisk server too complicated or difficult, there are some windows based options available.
I have recently been playing around with Call Butler, which was at one point a commercially available product that was offered for sale for $200. It is now available as open source software, and free of charge. Set up is quick and easy. If you are looking to set up a PC based PBX/IVR then i would recommend downloading this program and testing it out.
NCH also has a simple to use windows based PBX called Axon Virtual VoIP PBX. They also have a host of other telephony software available. This software is commercial software, with a trial period. You can download and toy around with the software to see if it will meet your needs, and then decide if it is worth the purchase price. I like NCH as they seem to have software for just about any business needs that you have. I have used the free version of there Express Invoice software for providing customers with a printed bill, but that software has so much more features. I would compare the Express Invoice software to Quick Books, or other accounting software.
[EDIT 11.23.10]
Phonebooth.com provides free hosted IVR service with 200 included minutes per month, and additional minutes are not too bad. There is also a monthly plan which includes VoIP service.
