Might it be that its not actually a virus that is changing the file? but rather mal-ware? There are a couple of programs that is known to do this.... Gain.Gator; nCase; etc...
You can download
Spybot here and check for any of these... (its free and fairly good, and it wont interfere with norton)
You can also check your computer free online
Here for any viruses that might have crawled past Norton (there is no anti-virus that is 100% secure) Do this especially when the CRC values of the files do not match the ones posted by Raze.