speaker producing unwanted noise

Some beginning troubleshooting steps:

1. You do not get this distortion plugging into any other device?
2. Do any other speakers have the same distortion plugging into your computer?
3. If your speakers have a volume knob, try lowering the sound on the computer output and increasing the volume on your volume knob and see if you get the same distortion. Do the opposite (High computer output sound, low volume knob)

When you said they worked fine for the first week and then you connected them to your PC. Does this mean that they were connected to another PC the first week? If they were connected to another PC you might want to reconnect them to that PC and see if they still make the noise and if you have a third PC, try them on it. If they continue to work on another PC, we can probably rule out the speakers being the issue. If they have the same issue on other PCs then it's probably the speakers. If we rule the speakers out as the issue then it could be one or more of several things on your PC. It could simply be the sound settings, or the sound card drivers need to be updated, or worse case scenario, the sound card is going bad. You could hook-up another set of speakers to your PC to see if they have the same problem. This might also help you determine if it's the PC or the speakers.

Good Luck!