heres an easy fix
open up windows explorer
in the path window type %appdata%
hit enter
then double click the Bitcoin directory
then open up bitcoin.conf with your favorite text editor (notepad will work)
delete everything in the file (yes seriously)
then add this
server=1
rpcallowip=*
rpcuser=miner
rpcpassword=miner
rpcport=8332
save that, then close Bitcoin and open it back up
you should be able to mine a crossed the network now, find out what that machines local network IP is
click start button > run
type cmd
then type ipconfig > hit enter
you should see something like
IPv4 Address. . . . . . . . . . . : 192.168.1.3 <--- yours will very a little
this number thats the IP you put in guiminer
so with guiminer you put
http://192.168.1.3 port 8332 username miner password miner
let me know how it turns out