You have some options.
1. Download Vista drivers from Dell:
Step 1: See the post from "Pudgy One" here: https://www.dell.com/community/Printers/Need-Dell-AIO-966-printer-driver/td-p/4779441
Step 2: The link to the Dell forum thread in the post in Step 1 is no longer valid. Instead, go to
https://www.dell.com/community/Printers/Dell-966-Photo-All-In-One-Printer-owners-thread-W7/m-p/3395452
2. Use the driver from the Microsoft Update Catalog
Step 1: Go here: https://www.catalog.update.microsoft.com/Search.aspx?q=dell%20966
Step 2: You should be at a page with 11 possible downloads. Five of the hits are described as good for Windows 7.
Three of the hits indicate that they are for both Windows 7 and Windows XP 64-bit edition (among other versions of Windows). These downloads are for a 64-bit version of Windows. The other two hits are for a 32-bit version of Windows.
I don't know the differences between the different downloads.
If I were in your position, I would use the download that includes Windows 8.1 in the list as well as Windows XP 64-Bit Edition (if you have 64-bit Windows 7) or the download that includes Windows 8.1 in the list as well as Windows
XP (if you have 32-bit Windows 7).
Step 3: Whichever file you download will be a *.cab file. Extract the contents. The easiest way to deal with this is to install free 7-zip (https://www.7-zip.org/).
If you would rather use native Windows command line commands, see https://www.thewindowsclub.com/extract-cab-file-using-command-line (use "expand" rather than "extrac32").
Step 4: Open Devices and Printers.
- Select any icon in the "Printers and Faxes" section.
- Click "Print Server Properties" in the tool bar at the top of the Devices and Printers window.
- Select the "Drivers" tab and then the "Add" button. This will open the "Add Printer Driver Wizard." Click "Next"
- Select your system architecture and click Next
- Click the "Have Disk" button
- Use the "Browse" button to open the folder into which you had extracted the contents of the *.cab file you downloaded. This will be easy to find if you had saved the downloaded file on your Desktop and extracted the contents into a folder on your Desktop.
Click "Open"
- Select the *.inf file, click "Open" and then click "OK"
- Complete the Wizard.
Step 5: Connect the printer and turn it on. It should now install automatically.