Hi padasalgi - Welcome to w7forums
I've taken a look and didn't see drivers for Windows 7. Try installing the Windows Vista drivers found here - HP Scanjet 3770 Digital Flatbed Scanner.
When Windows 7 drivers are not available, always try Windows Vista drivers. If the installer fails to start properly you can use Application Compatibility Mode.
Another thing to try is temporarily turning off UAC (User Account Control) while installing new Hardware and Software.
Configure Windows 7 UAC