unable to upgrade ios on iphone 3g

I finally downloaded ios 4.2.1 but when I try to install it on my iphone 3g the installation stalls on backup. The iphone backs up ok in the sync function in itunes and in fact was synced/backed up before i tried to install the new ios. The backup for firmware upgrade starts (barely) but can sit there indefinitely without progressing; ultimately I get fed up and cancel it. I tried closing and reopening itunes (which is latest version, PC), rebooting the computer and closing everything else on the computer except the relevant programs in case it needed more memory, but none of this worked. Any advice? Thanks.
iphone 3g iPhone OS 3.1.3