1) Make sure (if using wireless) that it is secure - try to change the key if possible (in case you have neighbours etc using your wireless).
2) Send your daughter out for the day,and turn OFF all computers and if possible connect your machine via ethernet (don't have your wireless broadcasting - see above). Wait 5 hours - this is the "time out" for any throttling Virgin may be applying. (If you time this right, there should be little disruption).
3) Turn on your machine with no internet type applications apart from your browser and check your speed
http://www.speedtest.net/ (it's not likely you'll be getting your full speed for various reasons). Use YOUR machine as normal, if speed is OK it could well be that Virgin are throttling your connection due to excessive use. If speed still no good, repeat test with another machine (to rule out a problem with yours). If same problem across multiple machines - have a moan at Virgin. Otherwise, if it only happens with multiple machines/ downloading happening, I suspect traffic management:
http://shop.virginmed...affic-management.html