Having installed Win 7 7057, I tested Abbyy FineReader and the app complained that twain_32.dll was not in C:\Windows. twain_32.dll is the driver for ScanGear, the scanning application bundled with my Canon AIO printer. So, I had to download the .dll file from the Net, and now everything is fine. Seems strange that the file is missing from OS.
I just went to the VISTA Windows directory and copied the twain_32.dll file there and pasted it into my Windows 7 windows directory. Fixed my scan problem and was painless and quick.