Dramatic drop in download speed

Hello, over the last 3 nights I've noticed that my download speed has dropped from the normal 7Mbps to less than 1Mbps and I don't know why.I've done a system restore to the 31st July but to no avail.Are there any checks I can do to see whether the fault lies with my PC, running Windows 7 or with TalkTalk.
Thanks in advance,
David

Dramatic drop in download speed