Thank you both again.

ROG, We've updated the Alesis driver (I say 'we' because my wife is rather more IT savvy than me and is helping with this). It said it was for all Windows versions.

We've also defragmented the disk even though it said it wasn't necessary at this time.

rharv, Yes, as you suggested we tried MME and playback is absolutely fine.

Finally, XP is Home Edition, Version 2002, Service Pack 3 !

I bought the computer just before Vista came out and it was sold as Vista capable. When Vista became available we updated to it but that certainly caused havoc with my music programs, so we went back to XP.

Incidentally, changing the buffer samples in the Alesis ASIO control panel appears to do little more than change the tone of the unwanted beep - the smaller the buffer rate, the higher frequency the beep.

Maybe we'll try going back to service pack 2 as our next step. Will let you know!