A few backdoor programs can work with any mail program, but not many. They do this by sitting in memory, waiting for you to send an e-mail. They then automatically attach themselves to any e-mail you send, again causing you to unintentionally send out malicious programs to your friends. This exploits a security flaw in Windows 9x, so no backdoor software is necessary. See About NetBIOS below for more information.
Different programs can do different amounts of damage.
This means that if a hacker can't do something using the backdoor program, he can easily put something else onto your computer that can. Hackers can see everything you are doing, and can access any file on your disk. Hackers can write new files, delete files, edit files, and do anything to a file that could be done to a file. A hacker could install several programs on to your system without your knowledge. Such programs could also be used to steal personal information such as passwords and credit card information.