It depends a bit on the OS and software you have installed and the connection. First, do you have the full software installed, if available for your OS? If you installed a generic OS driver, you should uninstall it and unplug the data cable until after the full software install requests it. Get the software here: http://h10025.www1.hp.com/ewfrf/wc/softwareCategory?lc=en&dlc=en&cc=us&product=60259&
and select your OS. (Newer Windows versions (like Win7) only use the driver in the OS; plug in the USB cable and let Windows Update find the driver.)
Put the document face down on the glass of the scanner or face up in an Automatic Document Feeder (ADF). Make sure the transport lock is unlocked on scanners with that feature.
If you have the full software installed, then start scans, at first, by launching the software (Start > All Programs > HP > HP Director or double-clicking the icon on the desktop). Then set the scan settings (Settings > Scan Settings and Preferences) - this is where you set the default destination and resolution, among other settings. You will see one setting for Button Settings. Once the button settings are set, if the monitor program is running, you will be able to launch scans from the buttons on the all-in-one. Click on Scan Picture or Scan Document depending on the item to be scanned. Adjust any settings and then press Scan. Check the preview if it comes up and then click Accept.
If the full software is not installed or you do not wish to use that software, you can use programs that are already in Windows or a third-party software. Windows XP, Vista and Win7 all include Paint, Start > All Programs > Accessories > Paint, which can acquire scans. Select File > Import from Camera or Scanner > Import from Scanner and then you will be able to scan. Vista and Win7 also include Windows Fax and Scan (Start > All Programs > Windows Fax and Scan). Click New Scan. The Windows Photo Gallery was preinstalled on Vista and is available as a download for Win 7 and can also acquire scans.
If you have problems scanning, try a new USB cable or a different port. If you are using a USB hub or print server, use a direct connection. (Print servers usually don't work for scanners). Scanners require bi-directional communication with the scanner and can fail even if the printer works.
I hope this helps.