A little clarification will help the diagnostic process, so let's get started!
I've used Twixtor on it for about 5 seconds.
Can you describe what you're doing in Twixtor? Since Twixtor's primarily a way to convert the frame rate of a clip, why are you only doing it for five seconds?
It's supposedly in HD.
Uh-huh. Why don't you know this for a fact? What kind of footage is this precisely? What are the H&V dimensions? What's the original frame rate? What's the codec? HINT: "mov" or "avi" is not a codec. It's a media container. Highlight the footage in the project window and get the straight skinny on it.
I have 2GB of Ram if you need to know.
Wow, that's almost NOTHING in AE-Land. Turn off multiprocessing if you have in on, and LEAVE IT OFF. You don't have anywhere near enough memory to use multiprocessing.
There's something else we need to know: the version of AE you're running. It wouldn't hurt to know about your computer, your OS, and any cool video cards, either.
Once we know all that stuff, we can start a diagnosis instead of guessing.
I'm just slowing down part of a clip with Twixtor.
When I say supposedly, it's because when I rendered it, it came out horrible. As I was trying to render it for the second time, it came out fine so don't worry. The only problem now is the time.
Ok, done that. Running CS4 and Windows 7. Not sure how I find out what my video cards are. Mind telling me?
Sorry, I can't give you a decent explanation because you didn't supply the information I asked for, so all I can do is speculate.
Here's what I think is going on,based on the available information: AE doesn't like you. What did you do to make it angry?