As-Salaamu Alaikum Wa Rahmatullahi Wa Barakatuhu,
I am not sure what you mean by "Video Downloader", but insha'Allah, I will try to provide some useful information. Please excuse me if my descriptions are oversimplified - I am ignorant as to how familiar with the subject others might be, so I think it is better to write such that nearly anyone can benefit, insha'Allah.
Insha'Allah, this will assist your Da'wah efforts. May you be rewarded in this life and the next for your desire to convey the true message of Islam to those who have been targeted by the centuries old, multi-billion-dollar campaign to indoctrinate non-Muslims, especially Christians, against Islam, exploiting and playing on virtually every animalistic, 'instinctive' human prejudice, irrational fear and hatred. I believe the most direct path to weakening these lower 'doorways' through which evil has always called to us, is by steadfastly rejecting them and actively seeking objective wisdom and understanding, which can only be granted by the Creator of all that exists. First, however, we must commit and conform ourselves to the path and instructions which the Creator, the Most High, Allah (Subhanahu Wa Ta'Ala) has revealed to humanity for its own benefit, by the messages and inspired lives of His prophets and messengers.
Please also excuse my long-windedness... Concerning a Video Downloader:
If you would like a program which allows you to type or paste a URL or internet address, which the program then downloads and plays (whether video or other media), I believe Windows Media Player (included with Windows) includes this feature. To use, open WMP, Click "File", then click on "Open URL", and insert the URL or address. If the media isn't 'protected' or encoded in one of a few proprietary formats, it should be possible to save (File - Save) the media to your computer for future use.
A very popular and far more configurable (and free) version of the standard Windows Media Player is "
Media Player Classic" (click to download). This program offers many, many options and functions (sometimes too many for some users), but those who enjoy flexibility and fine-tuning will, insha'Allah, like MPC.
You mention that you're looking for an iMovie-style program. Not being familiar with Mac software, I looked this up and it appears to be a movie-making or video-editing software. If you use Windows XP, there should be a built-in "Windows Movie Maker" which is very good for simple editing, inserting titles, captions and effects into most video files. I don't know if it is only included with WinXP Pro or also in Home edition (or Windows ME), but if you don't have access to this program for whatever reason, you may wish to look at the following free video editor:
Bobyte AviTricks
I haven't used this program myself, so I can't comment on its ease of use or features. I can't find any other free video editor, but most of the payware editors offer a trial period, if a short period of use is enough for your needs.
Insha'Allah, this will be helpful for you.
Wa Alaikum As-Salaam Wa Rahmatullahi Wa Barakatuhu