Microsoft refers me to article kb976982! Reading it, and with my limited techical expertise this seems beyond me. However, reading PC Advisor forums and other forums, it appears that this .NET Framework problem is an ongoing nasty headache for lots of us. I think that I will go down the route suggested by several irate posters and reinstall Vista - or go the whole hog and install Windows 7. I suppose I could always wait for Windows 8 and buy a new pc.
Try a system restore first. Go back to a time windows update was OK. Change how to update to 'just notify me'. When notified install one update. Do the same again until you're updated. Dot Net updates can be a pain and sometimes take a while to install even though you think the PC isn't doing anything. Whatever you do, do not cancel any updates, concerning Dot Net Framework updates, through being impatient. I learned that a few years ago.
So Afraid, make sure you only have windows update to notify you of updates. Don't allow to download and notify just notify. Once you get the yellow icon click on it then untick the boxes for the updates in the pane that opens. You will then get the option to hide the updates, do that.
Next run windows update and that should show you have no updates pending as they are already installed. Leave the updates hidden as you already have them installed.