Some detail: I have a Dell machine running Windows XP. It's about 5 years old. The ati2dvag dll is dated 2005, so is not a new version. Updating the driver using Catalyst did not find a newer version. Reinstalling the driver has no impact on the problem.
I read somewhere (possibly here) that vertical lines on the start-up screen suggest a hardware problem. Looking inside my PC, nothing appears to have come loose or to be damaged, but I can't tell for sure. There was quite a lot of dust in the fan inlet. I am wondering whether it could be that the fan is no longer working and the graphics card has overheated, but again I can't tell by looking whether this is the case. How would I be able to test this? One last thing: assuming the graphics card is the large component with the fins on (I don't know what it should look like), it doesn't look as though it can be removed like the RAM or other components can - it doesn't have the clips on each side that snap open - so if it comes to replacing the graphics card, I don't know whether that is possible.
Hopefully it's not going to be necessary to replace the whole machine. Any assistance and guidance you can offer will be very welcome. Thanks.