I don't know how they did it, but some person(s) managed to SPAM my category pages with links to their pages and such. I couldn't figure out how they did it, but when I rebuilt my entire blog, the SPAM disappeared. Did someone hack MT or somehow gain access to my login? No one has my info unless they hacked it... :/

Anyway, I am using Version 3.2. ANy thoughts on how to prevent this from happening again?

If the links were removed when you rebuilt your site, that indicates that it was the HTML files on the server that were modified - this could be done without access to your Movable Type installation (requiring login) or your database. I would recommend you contact your webhost about this, as it would indicate that someone on your server, or who has gained access to your server has changed the files.

You should also investigate whether or not you are able to set tigher permissions on the files published by Movable Type, which will reduce the likelihood that they will be vulnerable to hackers in the future. For more info about this, see this Learning Movable Type article.