Normally the Scan To button on the printer needs an application on the recieving machine to send the document to. In the past that was the HP Toolbox application. HP seems to be drifting away from the requirement of that application on future models which I believe is a good thing.
Unfortunetly for you if there is no Toolbox available for Win7 but there was for WinXP then chances are you will lose that functionality with the new OS. If HP was going to go back and develop a solution for your printer they would have done so when Win7 rolled out like they did for many other models. You will have to make due without the use of this feature and rely on normal scan applications like the HP Scan app. There are also many other 3rd party scanning apps that can comunicate with your scanner assumign the drivers were installed correctly after the upgrade.