I had exactly the same problem as the OP. I tried several of the solutions people have posted here, so thought I'd just share how I got on with those in case it's useful for any future readers.
I've lost the CD that came with the scanner, so I couldn't try dw96's first solution.
The VueScan program worked brilliantly. Unfortunately though, this isn't free. There is a free trial version that I downloaded which let me see that it worked, but this puts a watermark on the scanned pictures. The basic version costs about $39 and the professional version about $70, with the basic version only being entitled to upgrades for one or two years (I forget exactly) so that the professional version looked to be the better option. This was just a bit too much for me; I would have gone for it if it was under $20 (I think $10 would be a fair price) but as it was decided against it.
I tried downloading drivers for other Perfection models as suggested here and in other forums I found, but couldn't get my computer to recognise them as suitable for the 640U at all so that didn't work.
In the end the best solution for me was Linux. Obviously changing your whole operating system is a bit of a drastic solution to this problem! But I was planning to install Linux on a dual boot anyway, so this just made me get to it a bit quicker than I might have done. The Epson site has drivers for Linux; it was a little unclear just what to download out of the many files but I figured it out ok and it works fine! And completely free.