After defragging my hard drive, checking for viruses and spyware, removing any unnecessary start-up items and many other (unsuccessful) attempts to fix this issue, I found the following link.

Getting back to DMA mode in Windows XP | Sniptools | Miscellaneous

Basically, for any kind of respectable audio performance in Windows, the hard drive controller needs to be set to ‘DMA’ (direct memory access), rather than ‘PIO’ (programmed input/output).  You’ll see from the link that there are two possibilities, one involving a registry hack, and the other a few fairly straightforward mouse clicks.

Thanks to sniptools.com for getting me listening clearly again!