If you are running older versions of software with known security issues or have failed to upgrade your anti-virus software, the odds are better than ever that your machine has been infected with some form of malware
Hackers can install programs on your computer that give them access to your data. This malware often disguises itself as legitimate program so it can work undetected. View your list of installed programs from the Control Panel and look for any software you don't recognize.
If someone hacks your computer or if malware is installed, you may notice changes to your computer's settings. Your browser's home page may change, you may see some extra toolbars, new icons on your desktop or even a change to your online profiles.
If you're unable to log in to your online accounts, your passwords may have been changed.
This is especially easy for hackers to accomplish if they gain access to your email account, which they can use to request new passwords for your other accounts.
Pop up messages claiming that you have a virus and you are in need of anti-virus software may, ironically, actually contain a virus that could harm your computer, cause costly repairs or, even worse, lead to identity theft.
You start losing control. A computer virus can redirect your browser activity and prevent you from reaching websites you want. That's certainly the work of a hacker who came and went.
You start receiving odd email messages. Friends start telling you they've received strange emails from you (that you never sent), or you notice messages in your Sent folder you didn't write...these are clear signs a hack has likely happened.
You notice strange browser activity. Some hackers move right into a computer and control it as if they're in front of it.
If you ever notice browser activity that isn't yours (or someone else's in your family), that could mean trouble.