how to extract 512 bytes from a file

[Thomas Farrell]

      I am trying to recover some files which were on a windowz system,
which crashed go figure so someone wiped out the drive format and then
relized that they had important files on the drive. They then used sometype
of recovery software to extract them and now I have them. These files are
supposed to be AVI format but when I open them in a hex editor and extract
sections they are not part of an AVI file.  It looks like the recovery
software just read entire sectors and called them an avi file. I can clearly
see the html of cached web pages, and pointers to applications. I do have
some files that seem to be almost intact and in the correct format but the
header preamble of the file is missing . I was thinking I could take a
working avi and extract the first 512 bytes and insert it into the beginning
of a bogus file. Does anyone think this may work ? Any idea how I can
accomplish this ? I have tried with my hex editor extracting the first 512
bytes to a file like 1st512 and then  "cat corruptfile >> 1st512" and then
"mv 1st512  goodmovie.avi" but this is not working.  Am I wasting my time ?
Do I smoke too much weed ? Is this the wrong list for this question ?