First, make sure you have the latest FFDShow, just Google it.
Now, open your FFDShow config box, by going "Start > All Programs > ffdshow > Video Decoder Configuration"

Now, you need to make sure that the filters you are using are top priority.
Select "Blur & NR," from the list down the left, and hold the mouse down on it, and drag it up the list as far as it will allow.

Now, do the same for "Resize & aspect," so that it is just below "Blur & NR"

Now that they are where they should be, lets configure!
To start, click the box next to "Blur & NR" on the scroller on the left side.
Second, you will want to click the box next to "denoise3d" at the bottom of the config page.
Then, you will want to slide the first two sliders so they are both at "7.00"
Slide the last slider so that it is at "5.00" and tick the box that says "HQ"

Now, to change the "Resize and aspect," settings.
Again, click the box on the scroller on the left side that is next to "Resize and aspect."
Then, in the config window, click the option to have a "specific size." and enter in there what ever resolution you want. (I'm using 1280 x 720 [720p] for this example, although if your screen and CPU can support, 1920 x 1080 [1080p] would look awsome! And some people like to use 1440 x 960 as it is double DVD resolution)
Also, make sure that on the bottom of the configuration page, the aspect ratio changer is set to "No aspect ratio correction"

Next, go to the "Settings" tab just under "Resize and aspect."
There, in the drop down menu next to "Luma method" choose Lanczos. (You can change this if playback is too slow, if you prefer another method)
On the slider directly to the right, set "Parameter" to 10.
And set both "Luma sharpen" and "Chroma sharpen" to 1.50 each.
Now, click "Apply" and then "OK" to save your settings.

Now to test, open a video that uses FFDShow to decode, and see the quality difference!
Here are some examples, from Prison Break 1.01, Victoria's Secret Fashion Show 2005 and the intro scene of "Cache," a French movie.
(NOTE: All pictures are taken from same source [resolution at 640 wide], with the one on the left being original, the on the right being upconverted)

