Thank you for your reply! I updated the driver long ago, but it caused my laptp to keep locking up and eventually I had to disable the card altogether. Despite disabling the card, the stripes still showed up a few weeks later! I found out, since posting this here, that this is likely to be a graphic card failure and that Dell Vostro 1510 models made around the same time mine was, are all doomed to failure! Maybe I can get them to fix it, despite being out of warranty, seeing as it was destined to fail right from the start!
Many thanks
Amanda