Sound like the computer thinks it has a US keyboard fitted. Shift + 3 should give a � character on a UK keyboard but will produce a # on a US one.
Have a look here and scroll down to see the various layouts.
On his computer's 'Help' page, type in 'keyboard'. There should be a help subject on selecting the keyboard he is using. In Winxp, scroll down the Index to 'keyboards - changing keyboards and keyboard layouts', and follow the instructions.