Igot REALY old speakers that used to worke perfectly well

The speakers themselves are probably not your problem. Some speakers use the USB connection while others use the Audio output jack of your sound card. The problem is likely with your sound card (assuming you use the Audio output jack). If you rebuilt your computer, chances are you're missing the device driver for your sound card (these are small programs that let your hardware talk to your operating system). You'll need to download the correct device driver for your exact sound card. You probably don't know the sound card make/model. You can download Belarc Advisor (www.belarc.com) which will scan your PC for all hardware and software. Once you have that report, you should be able to go to the sound card manfucturer's website and download the latest device driver. You can always find out if it's the speakers by connecting them to another computer. Good luck.