Daniel Radcliffe
c/o Harry Potter Productions
Leavesden Studios
P.O. Box 3000
Leavesden
Hertfordshire WD25 7LT
England

Or at this address:

Daniel Radcliffe
c/o ARG Talent
4 Great Portland Street
London WIW 8PA
England

They say that if you want a reply, you need to make sure you put a return address, and make sure that the writting is legible. I already sent him a letter!