Thanks for your replies GhislainG and eibgrad.
GhislainG: I've now tried using the Google public DNS (22.214.171.124), but the problem's still the same- I assume this is what you meant?
eibgrad: When I ping sites I don't always get an IP address of 127.0.0.1
Oh yea, I've also tried loading safe mode with networking- but still no luck.
Any other ideas?