Martin 53N 1W wrote:

Do you have a lot of disk activity while this is happening?

No, hard disk LED flashes briefly every 5 seconds, but this happens no 
matter whether boinc is running or not.
 

Are you running USB or USB attached devices?

I have two USB ports, one for the printer, one for a digital camera.

Are you using a 'win-modem'?

No, I have an external Zoom modem (I'm on dialup).

Are you using a graphics chip on your motherboard, or a graphics card?

I have a graphics card (NVidia).

I also have a network card which connects this machine to another 
running Windows, but the latter is currently turned off.

BTW it seems boinc is the culprit since the problem occurs even if the 
seti client is not running (due to lack of work) as happened today.