My desktop speakers make noise when they are ON and not used

Besides eliminating interference, another suggestion:

Turn down the sound card/chipset volume from 100% to 75%.

Turn down inputs to the sound card/chipset from 100% to 75%.

Then, use the speaker's amplifier to compensate for any deficit in listening volume.

Why does this work? Amplifiers amplify, and when there is no useful sound to amplify, they will amplify noise. By turning down the sensitivity of their inputs and the volume of their outputs a bit, you'll cut the noise a lot.

In addition, it may improve your audio sound quality. When everything is set at or near 100%, a form of distortion called clipping occurs. This is when the tops and bottoms of sine-like waves get cut off flat, because they exceed the capacity of the amplifier (or sensitivity of the pre-amp inputs, etc). The result looks more like a square wave than a sine wave and sounds more like a square wave, too.

It's definitely not a virus issue.

As the comments say, it's in all likelihood normal electrical interference. It's quite common in unshielded audio/speaker cables.

Try moving the speakers/woofer around a bit, hopefully you'll find a place that minimizes the interference.

Another thing you can try is to update any audio drivers you have, there is a small chance this could help.

I remember having this problem on windows XP, what OS are you running ?

on Xp, I simply get into the "Sounds proprities" and mute all the others devices (wave, Mic ...), make sure your speakers are set to the appropriate speakers type and your micropfone is not using any "gain boost" or such feature.

Are you sure it's the speakers themselves and not the computer's sound output? I've been finding that since sound has been included on motherboards (instead of requiring add-on boards) that this is common occurrance. For whatever reason, it seems that on-board sound picks up a LOT of interference and sends it out the speakers/headphones. Especially if you've got the cpu set to be able to idle down to less than full speed.