If a hacker breaks into your computer, just noses around, and makes no changes to your computer, it's not easy to tell he's been there. There's no alert that says, "A hacker was here at 9:16 p.m. He works at the joint where you order your favorite pizza. His phone number is..."
In fact, Windows has no built-in log file that tracks events that occur on your computer. So it's possible that someone can come onto your system, roll around in your data, and leave without your knowledge. If you have a home network router or software firewall, your chances of suffering a hacker attack are massively reduced.
Signs of a hacker can include new odd behavior that suddenly starts to happen on your computer, such as the following:
The appearance of strange data, perhaps files or programs stored on your computer. A hacker might be using your computer as a temporary storage area or repository for pirated software.
Your computer might start sending spam. A hacker could have added software to send spam from your address.
You computer might start sending large volumes of garbage data to another computer. The hacker might have turned the computer into a zombie that can be commanded from afar to attack another computer in a distributed denial of service (DDoS) attack. In these attacks, bad guys harness thousands of computers to attack a single computer to overwhelm it with data and make it crash.
The first sign of a hacker could also simply be theft of money from your bank account. If you keep your banking information, including user IDs and passwords, in a file on your unprotected computer, it's entirely possible that someone could sneak on, find this information, and disappear again. However, phishers , key loggers, and other data capture schemes could also be responsible for this.