Hmmm...could be a few different things Pete, bit of a weird problem.
Which Operating System are you on? Is your Antivirus and firewall working properly and..under the task manager are you seeing any strange looking processes running that you can't account for (unsure if you are familiar with this bit) - this is where I first check as soon as something strange happens.
Have you had any other issues at all? has this suddenly just started? Is it after you have installed any other software - there may be a conflict somewhere for instance.
There's a few questions to start
_
TobyH