Well I've just transferred all of the iTunes music files back onto my laptop, and disconnected the external HD. My download rate has increased from 0.16Mb to 1.8Mb! I just downloaded some stuff off iTunes, it did 21Mb in under 2 mins. So, I haven't a clue why, but that external HD was screwing up my internet. Today's lesson: external HD is only for stuff I don't need to be using regularly!
Thanks for the help guys, I'm gonna start saving for a new machine now.