Time machine is a nice built in feature, but with the release of Lion, Apple enabled a feature called Local Backup This is a nice addition as it allows you to revert or undelete files without having your external hard drive connected. The downside to this is that it will make your system run slow if you've added or modified a lot of file between backups. Disabling local backups isn't as easy as you would think, and it should be as simple as unchecking a box in Time Machine's preferences. To enable or disable local backups you will need to open the terminal and run the following command.
To disable local backups:
$ sudo tmutil disablelocal
To enable local backups:
$ sudo tmutil enablelocal