My HP EliteBook 850 G1 was preinstalled as Windows 7 pro and includes a Windows 8 DVD. Since I am using dual monitors and Windows 8 provides extended taskbar for the second monitor, I decided to upgrade Windows 7 to 8.1. After I installed Windows 8 and many times Windows updates failure and reverting installations, finally I was able to upgrade to Windows 8.1.
When I enjoyed the Windows 8.1's extended taskbar and beautiful display themes, I found that the fingerprint login no longer worked. Google search showed that so many HP notebook users have the same problem.
I called the HP support and the technician from Mexican told me that I need send my computer to their service center for them to reinstall Windows 8. That was not my choice since I use this computer for my daily work.
Finally I figured out the problem and fixed the fingerprint login problem. Hopefully this post can benefit you for the same problem.

Here is the steps to fix the problem.

1. Download and install the latest "Validity Fingerprint Sensor Driver" from HP's website http://h20566.www2.hp.com/portal/site/hpsc/template.PAGE/public/psi/swdDetails/?sp4ts.oid=5405373&spf_p.tpst=swdMain&spf_p.prp_swdMain=wsrp-navigationalState%3Didx%253D%257CswItem%253Dob_134806_1%257CswEnvOID%253D4158%257CitemLocale%253D%257CswLang%253D%257Cmode%253D%257Caction%253DdriverDocument&javax.portlet.begCacheTok=com.vignette.cachetoken&javax.portlet.endCacheTok=com.vignette.cachetoken

2. Uninstall HP Client Security Manager and its dependent installations.

3. Run gpedit.msc to enable "Allow the use of biometrics" and "Allow users to log on using biometrics"

4. Download and install "Validity FingerPrint Reader 495 Driver" from Dell's website.
http://www.dell.com/support/home/us/en/19/Drivers/DriversDetails?driverId=GCKMY

5. Go to Windows accounts setting screen to add your finger scans.

6. Restart Windows and test your finger scan login.