This is the most likely cause (and solution) - click here and follow the advice carefully.
Another method of fixing the same problem - click here
See this thread also, particularly messages 5 and 7 - click here
Don't feel that you are being 'fobbed off' - the links above may look a bit complex and daunting at first, but if you print them off and read them carefully, they should start to make sense.
If not, feel free to post back (continue posting into this thread, using the message box at the bottom of the screen), and we will be able to help further if required.
Try to be specific, however, if you need further assistance. Try to describe which bits don't make sense to you, or why you feel that the solutions above don't apply to your specific circumstances.