I didn't seem to be receiving many stations on WSPR compared with the reports I was getting. I thought it might be because there was so much noise. But often I would get just one station for hours on end. I think it must have been my PC's time clock drifting because after I set the clock to update from the internet once every hour, I started receiving lots more.
I used this program to change the update interval from weekly to hourly.
Last night I had another go on the Samsung N130 netbook which I'd borrowed recently. Every 30 seconds when it's running on external power, the CPU usage spikes up and everything freezes for half a second, including the sound. On battery power, this doesn't happen. I've searched online and it could be something to do with the WiFi. This one is being used only on mobile broadband so the wireless could probably be disabled for now.