Assuming Windows 7 is fully updated (SP1 and IE10 whether or not you use IE - McAfee does) and all browsers and add-ons are also up to date this shouldn't be happening as I just checked BT NetProtect Plus system requirements and there shouldn't be any conflict. The only problem that would be obvious to me would be if you had a low-end CPU such as an Intel Atom - the software will not function properly on that.
try running the Virtual Technician to see if it fixes anything: http://mvt.mcafee.com/
Now there is a chance, because you are using BT NetProtect branded software that it wont help, in which case I suggest you contact Technical Support directly. It's free by phone or online chat and linked under Useful Links at the top of this page.
Thanks Ex-Brit. I got there in the end. Spoke with BT Security after McAfee pointed me in their direction. They seemed to believe the issue was because I was using an old version of Firefox (not really true as it upgraded when we checked the version from 24 to 25 - which had been released the day before) and they suggested I used Google Chrome. However from the conversation they suggested I could ignore the error messages as it probably related to something hanging around on the system causing the message to appear. That didn't convince me, but I ran the Virtual Technician which identified 9 issues and fixed 8. This improved the performance and I then ran CCleaner which cleared some junk.
Everything seems to be OK, so thanks for your help, although I'm still not sure I quite understand exactly what solved it.
Well I'm glad it appears to be OK now and I can quite understand when you say you aren't really sure what solved it. I find that often is the case with many things.