If you're lucky, you can go to your Start Menu folder and there should be a program called Profile Manager in the Thunderbird folder. After you run it once and create at least a second profile, then you'll get a profile selection box that comes up every time you start Thunderbird.
If you don't have that option, just edit your Thunderbird shortcut to /whatever path/Thunderbird.exe -ProfileManager, or Thunderbird.exe -P. That should bring up the profile manager, and then you can edit the shortcut back to normal after you've created the required profiles.
That should be fine if all you want to do is give everyone a separate profile. If you want to keep other people out, you'll need an extension like
ProfilePassword to password protect your profile. Of course, this is fairly weak protection, but it should be sufficient unless you have some techy friends who really want to read your email.