This is probably the best way to launch an argument. Currently Ubuntu is probably the simplest to get up and running. However, many people will probably recommend a different distro.
No matter what OS you install, as long as it's not Windows it won't be vulnerable to Windows attacks. Linux, for example, will be theoretically vulnerable to Linux attacks, however there are few in number and they generally get patched before someone can take advantage of them. If you want the maximum in security that would probably be one of the BSDs; however, bear in mind that there is less software available for BSD than for Linux.
TrueCrypt can be used to lock a partition, a backup file, or an encrypted container for everyday use. It's pretty simple to use, and you can even set it up to lock your home folder (which then gets automatically unlocked by your password when you log in).