Ahh found it!
I had exactly the same problem with my 3400, so I guess something has
changed post Vista SP1. However I worked around it... the above fix is
fine to an extent, but you need to add an extra step:
1. Download the XP drivers from the HP website.
**not needed**
2. You have to be running as administrator. (Check this under Start /
Settings / User Accounts - you can see if you are administrator from
there. If not, change your account type to administrator.)
**not needed**
**definitely not needed**
3. You have to turn off UAC in Vista. See
'Disable User Account Control in Windows Vista'
(http://www.petri.co.il/disable_uac_in_windows_vista.htm) for details
about how to do this.
**definitely not needed**
4. Find the driver you downloaded from HP and unzip. Don't just run it
(it's an executable zip file). Just unzip it as if it were a regular
zip file first. Then find the setuo.exe executable file. Right mouse it
and select properties. Select compatibility and click XP service pack
2.
Store and close.
**this is the step you need to change... I'm assuming you're running as
a standard user without admin rights (if not you should be... more
secure!), right click on the setup.exe file you extracted and go to
properties. At the bottom of the window, select the 'show for all'
option, and then set it to run as XP SP2 compatability.**
5. Run setup.exe. The driver software will install.
6. Connect the scanner.
7. Don't give any more money to those ******** at HP. honestly, to tell
a customer to buy a new scanner instead of using runas and compatbility
mode is bloody shocking.
PS: sorry if this is a necro in some peoples eyes; I did a search for
this problem but didn't realise how old this post was before replying.

Ahh found it!
I had exactly the same problem with my 3400, so I guess something has
changed post Vista SP1. However I worked around it... the above fix is
fine to an extent, but you need to add an extra step:
1. Download the XP drivers from the HP website.
**not needed**
2. You have to be running as administrator. (Check this under Start /
Settings / User Accounts - you can see if you are administrator from
there. If not, change your account type to administrator.)
**not needed**
**definitely not needed**
3. You have to turn off UAC in Vista. See
'Disable User Account Control in Windows Vista'
(http://www.petri.co.il/disable_uac_in_windows_vista.htm) for details
about how to do this.
**definitely not needed**
4. Find the driver you downloaded from HP and unzip. Don't just run it
(it's an executable zip file). Just unzip it as if it were a regular
zip file first. Then find the setuo.exe executable file. Right mouse it
and select properties. Select compatibility and click XP service pack
2.
Store and close.
**this is the step you need to change... I'm assuming you're running as
a standard user without admin rights (if not you should be... more
secure!), right click on the setup.exe file you extracted and go to
properties. At the bottom of the window, select the 'show for all'
option, and then set it to run as XP SP2 compatability.**
5. Run setup.exe. The driver software will install.
6. Connect the scanner.
7. Don't give any more money to those ******** at HP. honestly, to tell
a customer to buy a new scanner instead of using runas and compatbility
mode is bloody shocking.
PS: sorry if this is a necro in some peoples eyes; I did a search for
this problem but didn't realise how old this post was before replying.