This works on Windows 8 64bit but follow the instructions I have written.
The drivers are Epson, they are free, they are easy to install (if you follow my instructions) and they work on the Epson 1640SU. When you get to the page that allows you to choose your driver, there is one for the 1200 series; either 1260 or 1270 I can't quite remember.
Anyway, see below:
Go to the following Epson site & download file 12180 (if the link doesn't open just highlight, copy and paste into your browser):
http://www.epson.com/cgi-bin/Store/support/SupportDownloading.jsp?download=ftp%3A%2F%2Fftp.epson.com%2Fdrivers%2Fepson12180.exe&pc=Epson+Perfection%3Csup%3E%AE%3C%2Fsup%3E+Series&pn=Epson+Perfection+2400+Photo&ft=driver&returnpage=%2Fsupport%2FsupDetail.jsp&returnparams=%26infoType%3DDownloads%26oid%3D14587%26prodoid%3D21643926&licenseoid=17774&BV_UseBVCookie=yes&why=100&fnbjm=&contact=00&submitForm.x=35&submitForm.y=7&submitForm=01&ref=r014001222
Using Winzip or whatever, extract and install the file as per default settings.
Connect you Epson Scanner and go to Device Manager in Control Panel. The scanner will be shown but will have a marker against it. Right click and select update driver.
Select "Browse my Computer" then "Let me pick list of device drivers from my computer" (don't select or let the computer search for drivers because it will not find any).
Choose "Imaging Devices" and then click "Have Disk".
Navigate to the folder that you have just unzipped and you will be offered a selection of Epson Scanners. Choose the Epson 2400 and select install. Ignore all the warning etc that Microsoft throw up.
The Device Manager will now show that you have an Epson 2400 scanner installed rather than the Epson 1640SU but that doesn't matter as your 1640SU will work perfectly well with the twain driver installed - including on 64bit systems like Windows 8.
That's it and best of all; its free.
Steve