I use fraps (
http://www.fraps.com/) to capture clips of interesting moments from the ingame replays. I think you need to buy the registered version to unlock the maximum resolution and not have their logo overlayed, but you can play around with the free version to see if you like it first.
The clips fraps records are the same quality as in the game, but very big - typically a few gigabytes for a minute of action. Next I import them into Windows movie maker. I'm using version 6, I think there are other versions that will do the same thing but maybe options are called differently etc. In movie maker you can do any editing required, if I'm feeling creative maybe cutting together some action filmed from two different viewpoints etc. I also add effects, normally just fading in and out at the start and end just for a little polish.
Once the movie is edited how you want you click on 'publish' it. The settings you choose here will effect how much compression will take place and thus the resulting filesize. There are many different types of compression, but the general rule is that the more the film is compressed the more quality is lost, so it's a tradeoff between the two.
You should end up with a video that is a managable size for upload/download, typically around 10-50MB depending on the length of the clip. You can upload this to youtube and then use the link in your forum posts and it will embed the clip in the post
