I've just been to the site and expanded the vista 32 bit driver
and it seems as though the driver is not a signed driver, and therefore wouldnt install on Windows 7 64BIT OS you could try the Twain driver version 5.53A it seems to be a signed one - to make this possible you will have to install it in compatibility mode with xp otherwise it wont install, because of a OS mismatch, also note it says on the site the scanner has to be disconnected from the pc while installing the driver, anyhow to install the driver do the following :
To install a driver OR program in compatibility mode - right click the setup.exe file - click properties - click compatibility - check run in compatibility with vista/xp
etc - check run as administrator - leave the rest blank. Start setup.exe and install.
Otherwise you will have to turn off signed driver checking which is not recommended - you can find instructions for this by typing something similar to this in the forum search .
I have just found out 32 bit drivers
wont install on a 64 bit OS in any case