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In some situations, you may see new programs or files on the computer. If you are the only user on the computer and new programs are installed, this could be an indication that it has been hacked. However, there are also several legitimate reasons why a new program may appear on the computer, as listed below.
Operating system or other program received updates that included new programs or files.
When you installed a new program, other programs may have been installed with it. For example, it is common for plugins and other free programs to have a check box asking if it is ok to install a new Internet browser toolbar or antivirus program on your computer. If you don't uncheck these boxes, the additional new programs are installed.
If you suspect someone may have used your machine, ask if they installed a new program.
Below is a listing of programs that may indicate a hacker has been on the computer. Backdoors and trojans are by far the most common programs installed on a computer after it has been hacked. These programs can allow the hacker to gain access to a large amount of information stored on your computer.
IRC clients are another common way for a hacker to get into a computer or remotely control thousands of computers. If you have ever participated in an IRC chat, your computer could have been hacked.
Spyware, rogue antivirus programs, and malware might be an indication of a hacker. More commonly, however, they are a sign that your computer has been infected via download or visiting a hijacked page while on the Internet.
Computer passwords have changed
Sometimes, after an online account is hacked, the hacker changes the password to one or more accounts. Try using forgot password feature to reset the password. If your e-mail address has changed or this feature does not work, contact the company who is providing the service. They are the only ones who can reset your account and give control back to you.
Local computer password
If your password to log into your computer has changed, it may have been hacked. There is no reason why a password would change on its own.
Lost or forgotten Windows password.
E-mail spam being sent
Getting bounce back e-mails from addresses I don't know.
For any attacker to take control of a computer, they must remotely connect to it. When someone is remotely connected to your computer, your Internet connection will be slower. Also, many times after the computer is hacked, it becomes a zombie to attack other computers.
Installing a bandwidth monitor program on the computer can help determine which programs are using bandwidth on your computer. Windows users can also use the netstat command to determine remote established network connections and open ports.
Computer security programs and firewalls help restrict access for security purposes. If the computer prompts for access to programs you do not know, rogue programs may be installed or it may have been hacked. If you do not know why a program needs access to the Internet, we recommend blocking access to that program. If you later discover these blocks cause problems, they can be removed.
Tip: A firewall prompting you for access may also just be someone trying to probe your network, looking for open or available ports.
Security programs uninstalled
If the computer's antivirus program, anti-malware program, or firewall has been uninstalled or disabled, it can also be an indication of a hacked computer. A hacker may disable these programs to help hide any warnings that would appear while they are on your machine.
Note: It is also possible for a virus to disable the antivirus program or malware to interfere with the anti-malware program.
Computer doing things by itself
When someone is remotely connected to a computer, they can remotely control any device on that computer. For example, a mouse cursor could be moved or something could be typed. If you see the computer doing something as if someone else is in control, this can be an indication of a hacked computer.
Modem users
If the computer is dialling the Internet on its own, it is an indication that a program needs to connect to the Internet. It is common for programs like e-mail clients to do this to check for new e-mail. However, if you cannot identify what program needs Internet access, this could be an indication of a hacked computer.
These symptoms may also be an indication of a virus or malware infection in the computer. Viruses often attempt to establish an Internet connection to send data from your computer to a malicious destination.
Internet browser home page changed
If you notice that your web browser configuration has suddenly changed, this may be a symptom of virus or malware infection. Examples of sudden browser changes include your home page changing, a third-party toolbar being added, or your default search engine changing to something you don't want.
—computerhope.com
