Windows Defender came installed on my laptop. I downloaded and currently use Spybot Search & Destroy. Do they do the same things? Do I need both, or can I get rid of one of them. If so, which one?

By the way, I'm running Vista Home Premium.

If they are different, I'd like to know how they are different and what each of them does.

If I should keep both of them, are there parts of them that I should disable. For example, keeping Spybot but disabling Teatimer.

Ive got both, its worth having both, i think spybot is more effective at finding threats then windows defender.

Windows Defender is a feature of Windows Vista and part of the operating system so you cannot uninstall it. However, you can disable Defender.

"Windows Defender on Vista Explained"
"Windows Defender Technical Overview"
"Understanding Windows Defender real-time protection"

Using Spybot S&D
Spybot S&D Features explained
Advanced Mode, Tools, HOSTS file viewer, TeaTimer, SDHelper, System Startup