does anybody know how to make a slowmotion video?

I guess that that would depend on what you are recording. Some video editors are capable of increasing/decreasing the speed of the video, but depending on the video, this could make the video choppy. It is best to record at a high frame rate so that when you slow it down there is enough information to yield a smooth slow motion video. I use Adobe AfterEffects for time remapping and speed changes, but it can also be done on Premiere and Final Cut Pro. I don't know much about freeware video editors, but you might be able to find one that has that feature.

Slow motion shots of very high-speed objects like a bullet hitting an apple, need to be shot with very high-speed cameras (probably expensive).

I've used Premiere to do slow motion, but you can only slow it down so much before you start seeing individual frame passes. Like regular video is shot at 25 or 30 fps, and in something like 300 they shot the slow motion sequences on a 300 fps camera. Definitely not something in the average person's budget.

Might sound corny but depending on what its for you can slow down the actors a bit and it does the same thing as shooting at a higher frame rate. Then you just need to slow it down a little to get a "special effects" feel while still maintaining frame rate.

Takes alot of practice, we did it for a kung fu fight scene in a music video we shot for a drama class a few years back. It helps to work out the numbers (wasnt involved in that sorry) our guys had position points they hit when a guy called out time on a stop watch so they knew how much slower they were moving compared to 'real life'.

You can rent high speed cameras if your in a city with a rental company of that type. But they are expensive.

Good luck.
-Jordan

If your video is captured in avi or mpeg format (so not in Quicktime or wmv), then you can use VirtualDub. Load the video, do ctrl+r (or go to the video>framerate menu) and under 'source rate adjustment' you should select 'change to' and change the framerate so it matches the desired rate (if you want half speed, and the source is captured at 30fps, you'll have to enter 15)

Freeware solution for what you want would be Virtualdub. (virtualdub.org)
It's a handy avi/mpg editor with lots of options and filters. Best of all, it's free