Malware
Malware authors target bugs, or loopholes, to exploit. A common method is exploitation of a buffer overrun vulnerability, where software designed to store data in a specified region of memory does not prevent more data than the buffer can accommodate being supplied. Malware may provide data that overflows the buffer, with malicious executable code or data after the end; when this payload is accessed it does what the attacker, not the legitimate software, determines.
HijackThis
HijackThis (also HiJackThis or HJT) is a free and open-source tool to detect malware and adware on Microsoft Windows. It was originally created by Merijn Bellekom, and later sold to Trend Micro. The program is notable for quickly scanning a user's computer to display the most common locations of malware, rather than relying on a database of known spyware. HijackThis is used primarily for diagnosis of malware, not to remove or detect spyware—as uninformed use of its removal facilities can cause significant software damage to a computer. Browser hijacking can cause malware to be installed on a computer.