I have an old backup that includes many .exe files. lately I compared the content of the backup to the content in my computer and I noticed that all .exe files (and ppt) are changed. Using a comparison tool showed me that few bytes are changed at the beginning of the file and at the end of the file lots of bytes where added. Scanning the files using various antivirus tools - discovered nothing.
I'm still not sure that this is not a virus. So, my question - does anybody have a possible explanation? Do you know of some service or some specialists group that I can send them a sample of 'clean' and 'infected' files and they will analyze them?