Old film can have all kinds of artifacts in it and can decay to the point where it goes in and out of focus as you play it. While you can't add details to low-quality film, you can work to extract details hidden by noise. I'm just starting out with this and one of our video experts turned me on to a simple trick with brightness, contrast and sharpening.
Below is a clip from the 50s British TV series, The Adventures of Richard the Lion Heart. It's total schlock, from beginning to end. The DVD I got is foggy and messy. A little work with Adobe Premiere revealed a bit more details. You can see the processed version on the right and the original on the left. Here, the brightness is increased, causing some over saturation, but giving me enough light to boost the contrast quite a bit which goes a long way towards getting the gray fog out. Increasing the sharpness accentuates color differentials, giving you better demarcation on edges.
This is a start on learning how to make the most of poor quality video, but I liked the results so far.