Download the latest drivers from HP. All almost 200mb of them. Yup, that's what my girlfriend had to do with the free printer/scanner they gave her. I think it's the same model.

Good advice, except the OP has Leopard. Presently, the newest drivers for this all-in-one C6180 (as well as an enormous variety of other printers) are included with Leopard. The best thing is to remove any drivers that you may have downloaded from the HP website, and do a custom install of just the HP printer drivers from the Leopard installer DVD.
Here's some good info from the HP web sitehttp://h10025.www1.hp.com/ewfrf/wc/document?docname=c01234231&lc=en&cc=us&dlc=en&product=1153494&os=219&lang=en

I got it to work with Leopard 10.5.1.

Hi, I have the same problem with the same 4070 scanner but cannot really understand the 10 steps. Would you be able to explain them in a few more words please :) I have downloaded the .dmg and mounted it but the rest kind of went over my head.
Thanks a lot, Dylan

before installing other software try looking in the HP folder in applications
then go to Device Utilities, then HP All-in-One Device Chooser, and select the printer AiO you are using, if you are able to print to it it will show.

this worked for me and i am able to scan and copy without problems now

good luck

(merged two threads about same problem started by the same person)

philworrall, please don't start a second thread with the same subject matter as one that you previously started.

Trevor

OK, this problem bugged the hell out of me.
I finally found a personal solution for this.
I installed the Apple software update drivers for the printer.
Uninstalled the HP softwares and drivers (even though they said they're meant for Leopard)
Use Vuescan to scan.
And viola, it worked!
Not sure why, but after numerous installs and uninstalls of the HP drivers, I am glad something worked!
Yes, Vuescan is not free. Perhaps any 3rd party scan software would do.
Just uninstall those HP drivers from your computer. (Go to Applications, Hewlatt-Packard, choose the Uninstaller)