Firstly, you don't want Ubuntu. If you have the T60 version with Intel graphics, it may not boot. In any case, it's prone to all sorts of odd problems from version to version.
You want something that will easily encrypt your data, by ticking a box at installation time: Thinkpads are very stealable. You choice will depend on how much memory you've got. Good options are:
Linux Mint: Ubuntu debugged and improved. The Mate (plain) and Cinnamon (fancy) GUIs will both just about run in 512MB and will be great with 1MB.
Fuduntu. Rolling release: no versions, you just install and update regularly. Particularly aimed at laptops. Recommends a minimum of 384MB.
SalentOS. Very light and fast.
See the reviews from the links at Distrowatch or the ones here
PS. Do you know this site: very handy (I've got a Thinkpad myself)