How viruses enter your PC

Viruses have targeted various types of transmission media or hosts including, but not limited to, the following:

Binary executable files (such as COM files and EXE files in MS-DOS, Portable Executable files in Microsoft Windows, and ELF files in Linux)
Volume Boot Records of floppy disks and hard disk partitions
The master boot record (MBR) of a hard disk
General-purpose script files (such as batch files in MS-DOS and Microsoft Windows, VBScript files, and shell script files on Unix-like platforms).
Application-specific script files (such as Telix-scripts)
Documents that can contain macros (such as Microsoft Word documents, Microsoft Excel spreadsheets, AmiPro documents, and Microsoft Access database files)
Cross-site scripting vulnerabilities in web applications
Arbitrary computer files. An exploitable buffer overflow, format string, race condition or other exploitable bug in a program which reads the file could be used to trigger the execution of code hidden within it. Most bugs of this type can be made more difficult to exploit in computer architectures with protection features such as an execute disable bit and/or address space layout randomization.
Adobe PDF files that link to malicious code.
HTML (web page documents) that include malicious scripts or other embedded executables.

It is worth noting that some virus authors have written an .EXE extension on the end of .PNG (for example), hoping that users would stop at the trusted file type without noticing that the computer would start with the final type of file. (Many operating systems hide the extensions of known file types by default, so for example a filename ending in ".png.exe" would be shown ending in ".png".) See Trojan horse (computing).