Pick up an Authentec fingerprint reader. Windows can get the driver from Windows Update, and although Authentec got bought by Apple and doesn't sell software suites anymore, they've still got a basic one that'll let you do Windows log-on and elevation prompts. You can get that here, including the license key: http://support.authentec.com/Downloa...TrueSuite.aspx
Although it says Windows 7, that should work fine on Win8 too.
Authentec/UPEK made various fingerprint readers. Some are swipe sensors, some are stationary touch sensors. Their model name is Eikon. Looking on Ebay, I see some of the Eikon TrueMe models for $20 shipped. I have one of those and can verify it jives with the free software mentioned above. In that general swipe-reading style, the primo unit is the TCRD4. I have one NIB I'd sell for $45 shipped (USPS) if you wanted.
There are also lots of UPEK old-style touchpad fingerprint readers on Ebay, most of them transparent-blue color. They do NOT jive with the software mentioned above, and were glitchy with other software, sometimes acting as if they'd been touched at random times. The Eikon 300/500/700 touchpad style are a good bet if you want the touchpad style instead of the swipe style.
There's also keyboards incorporating Authentec/UPEK touchchips, if that works better than a standalone sensor. I've tried two types: a Cherry with a touchpad (had the glitching problem I mentioned above), and an IBM/Lenovo that works great. I have a half-dozen of the Lenovos at work. The IBM/Lenovos crop up on Ebay, part number 73P4730 for the U.S. key layout. You can also search for "fingerprint keyboard," but beware that there are other languages with alternate key layouts. Look at the shape of the ENTER key, if it's the horizontal rectangle then that should be English. L-shaped or inverted-L-shaped indicate other versions.
For those who'd like the Lenovo in Portuguese, Spanish, or French layouts, check Geeks.com.