If an engineer can't work out what the issue was (and I see from a different thread that he had no issues with his computer on your connection - which means it is likely to be your computer that is the problem) then I would advise you to take a backup of all of your documents etc onto CD/DVD/different hard drive partition, then reformat your hard drive and reinstall from scratch.
You need to make sure you have copies of (or can download) all of the programs that you frequently use so that you can reinstall these and obviously you need to have a copy of whichever OS you normally use.
To be honest when I used to use M$ at home I would do a full reformat and reinstall once a year as it seems to be the only way to keep things running at a decent speed and stable in the long term.