The type of virus that executes when a computer starts is called a boot sector virus.
Boot Sector Virus Explanation
A boot sector virus infects the boot sector or the master boot record (MBR) of a storage device such as a hard drive or USB drive. It loads and executes its malicious code before the operating system itself starts up. Because it runs so early in the startup process, even before antivirus software is fully active, it can be very difficult to detect and remove.
How It Works and Why It's Dangerous
The boot sector contains essential code that instructs the computer on how to load the operating system. The boot sector virus replaces or modifies this code with its own. When the computer powers on, the virus code executes first, often copying itself into memory and potentially spreading the infection. This early execution means the virus can compromise system security and disrupt normal booting, sometimes causing errors or total failure to boot.
Examples and Symptoms
Classic examples include viruses like "Michelangelo" and "Brain," which infected the MBR or boot sector and became active either on startup or on specific trigger dates. Symptoms might include error messages during startup such as "Missing Operating System," system crashes, or inability to boot into the OS.
In summary, the virus that runs when a computer starts is known as a boot sector virus , targeting the critical boot area to execute itself before the operating system loads, making it a serious and stealthy threat.