Caz, from what you and then Vacationguy have said regarding slow boot up, you'd do well to do Disk Cleanup and then Disk Defragmenter. My gf had an old PC given to her by her parents that likewise suffered very very long boot times when first turned on and going through these procedures helped a lot.
On most versions of Windows if you go Start Menu -> Accessories -> System Tools you'll see both of them. Run Disk Cleanup which might find a lot of old redundant temporary files. Then run Disk Defragmenter (and choose to Defragment drive C:, NOT just Analyze) which takes all the chunks of data on your hard drive and arranges them efficiently again - think of it as taking a messy filing cabinet and putting everything back into alphabetical order again, stuff is much faster to find that way.
Defragmenting can take many many hours (sometimes a day or two!) and you need to be patient and let it run!
As for BT and the telephone exchange box: no, the wires running into your house aren't going to be fibre optics themselves but that shouldn't make too much difference considering the box is only at the end of your street.
This is an article from June which seems to describe the same problem you're having. It doesn't offer a solution but they have been very thorough in eliminating everything but BT as the source of the problem:
BT Infinity does badly in real world speed test | KitGuru