Be sure you installed the Fingerprint reader driver, you can check your device manager to make sure you did. If you did, then try re-installing the Fingerprint validation software. Don't know what yours is called who the vendor whp designed it but for my HP Pavilon dm4 laptop, its called SimplePass Identity Protection and designed by Egistec.
Also try checking for soultions on the Dell website, maybe you could download the drivers there
I'm currently experiencing the exact same problem with a different model laptop. I believe it may have something to do with the BIOS. I don't know the exact model of the laptop you have so I can't direct you as to what file you'll need specifically, but check the dell website for a BIOS upgrade for the fingerprint reader. Is this by chance the AES2810?
Check device manager and see if Biometric Devices is in there.