Remember that Snow Leopard does NOT use the HP applications that Leopard used.
You should uninstal all the previous drivers and HP utilities and let Snow Leopard instal the current HP drivers. Then you will find the scan function is supported by Preview (file>Import from Scanner).
I have 13" MacBookPro running 10.6.1 with 4 G Ram. My HP Photosmart C4500 All-In-One will print but will not scan. I have unplugged the USB cable and uninstalled and reinstalled the driver to no avail. I have tried to scan from both the unit itself and the scan software on my laptop. I get messages like "an unknown error occurred, or "can't find scanner". When you look in the 4500s window it appears to be scanning but nothing happens. Any ideas? It seems to have stumped HP help so far!