regardless of how the source was originally created, if you have an interlaced capture file, unless you deinterlace it, it will be nigh on impossible to run filters on it to sharpen or improve the image further as most filters for avisynth and vdub are designed to work with a progressive non-interlaced sourec.
so even if it looks fine on your PC screen, it is still interlaced. to smooth, sharpen or do anything else you will need to deinterlace to get it back to a non interlaced progressive source file.
eg Star wars on LD PAL.
25fps interlaced file, deinterlace back to 25fps progressive.
Star Wars on LD NTSC
29.97fps NTSC interlaced file, IVTC back to 23.97fps progressive using deinterlace and frame decimation.
that's my read on this situation.