Capture VGA video through your USB port
Capturing video from a VGA source through your USB port isn't an easy as it sounds - it requires converting the analog video to a digital format which can then be stored on your hard drive.
If you want to convert home movies from your VCR, you're probably better off just buying a low-cost USB video capture adapter (note, however, that this won't capture from VGA - it will only accept composite or S-video inputs, which should be readily available with any standard VCR). If you need high-quality, high-definition images, however, the VGA2USB is pretty much the least expensive option available.
At $300 the VGA2USB is a big step up in terms of price (cheap models usually run in the neighborhood of $35), but difference in quality more than makes up for the price. Also, the included software gives you a greater range of flexibility.
Ways to capture
There are a few different ways to capture your video - first of all, you can view captured data in real-time. This is helpful if you are capturing medical device images, real-time video from a game or any other time that you would want to be capturing video while you're watching it.
Another option is to save captured images to a disk - this doesn't even require that you have a monitor setup. A common application for this is a security camera where you want to be recording the cameras output, but you don't necessarily want to monitor the output in real-time. You can save the video footage to a hard-drive and then review it later.
Storing video feeds does consume a lot of hard-drive space (you can change the settings to control the quality and hence the space required, however), so some people opt to just periodically take a screenshot of the output on the page. You can easily control how frequently snapshots are taken.
Here is another great review on the VGA2USB from Firing Squad, our favorite hardcore gamers.