Fingerprint Sensor driver needed in Dell Vostro 3550 Win7 64bit

That driver can be found here: Support for Vostro 3550 | Drivers & Downloads | Dell Canada
specifically, this Fingerprint Sensor Driver: https://dl.dell.com/Security/R294095.exe

Windows 7 is unsupported and insecure. It does not receive security patches, so it should no longer be used on a PC connected to the Internet.

I suggest that you update to Windows 10. Do the free upgrade: https://www.zdnet.com/article/heres-how-you-can-still-get-a-free-windows-10-upgrade/
After upgrading, check to see if Windows 10 is activated in Settings | Activation. If it is, then you can perform a clean install of Windows 10 and it should activate automatically.

If you do upgrade to Windows 10, then I would suggest that you install the Fingerprint Sensor Driver for Windows 8 (no Windows 10 driver is available). If you have issues installing it, then you can install it in Win8 Compatibility Mode.