Look for Hopper8's 'Odin won't flash' thread. Plenty of things to try in there. Also, check the rom package you've downloaded; either check the md5 sum if you have it (with md5summer or similar), or failing that, use 7Zip/IZarc/similar to 'test' the rom package (Test archive in 7Zip, test in IZarc). This will check that all the files in the package are OK/none are damaged or missing.
If the package passes either that test or md5sum check & you still can't get it to take a flash after trying everything in Hopper's thread, it's probably cactus.