I did eventually get this problem solved... I took it to a local computer repair guy!
He tells me that he un-installed Windows and then re-installed it with a version that came direct from Microsoft. He said that the Windows that came pre-installed on the laptop from Acer, was full of bugs.
I have no way of knowing whether this is true or not, but for £50 I got my brand-new laptop working properly, and no problems since then.