Viruses and Malware

I volunteer at my church and I have access to several PCs on a WPA2 protected network. My PC is running Windows 7 and Windows Media Player 12. We have a cable broadband connection and have approximately 25 PCs connected. At random times, and with no programs running in the background, I begin hearing someone talking through the speakers on my PC only. It’s not a radio station or similar media playing. For what is being said, and other sounds, it sounds like an audio feed from a porn site. If I let it play, it will run for about a minute or so and then it will stop. It may start again a few minutes later.

When this happens, I have immediately started Task Manager but I see nothing showing under the Programs tab. Next, I look at the Processes tab and don’t see anything unusual there as far as I can tell. I have asked this question on other sites and keep getting the same, “Well, you can’t tell me that you haven’t been to a porn site” response but I can assure you that I haven’t visited any site with this type of material. And no one else has access to this computer. I’ve checked Windows Media Player Sharing and do not have any type of media sharing turned on. I’m afraid that some here will begin thinking that I’m using the church’s network for “unsavory” uses.