If you have Win 7 Pro and the disk is NTFS, then EFS should be available.
From an administrative command line (right click on command prompt and Run as administrator), run:
fsutil behavior query DisableEncryption If = 1, then it has been disabled, if = 0 then there is some other reason it's greyed out.
If = 1, enable by running
fsutil behavior set DisableEncryption 0 then reboot
Do you have an SSD? You might find that it supports hardware disk encryption, but it's not easy to get that working under Win7 -
https://github.com/Drive-Trust-Alliance/sedutil/wiki
SD