Hi!
Thank you for your help. Your suggestions have led me to do several things.
I run an Intel Pentium 4 with 3.0 GHz wich for some reason appears twice in 
my hardware manager (Win XP home). I have tried deinstalling one of them, 
but Windows insists on showing it twice. After several attempts and reboots 
I have given up on this. I have however changed my preferences on the 
seti@home homepage to use just one processor on multi-processor machines. 
Though I don't understand why this works (unless it relates to what I have 
described above) it seems to have done the trick. My second work unit is now 
parked.
Best regards,
Martina