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.
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.