Although traditionally many computer viruses have been relatively harmless, being primarily concerned with staying in the computer long enough to be spread around, it seems that a growing percentage of the malware we see is designed to be highly destructive. Even many relatively innocuous viruses can cause your computer to crash, corrupt data, perform poorly, or just not function properly. Today, most viruses spread via e-mail and Internet downloads. Microsoft products such as Word and Excel support macros which are a very common place to find malware.
Postscript can contain executable code.
HTML pages may contain ActiveX, JavaScript, or Java code which can include malware, especially after it is on your local machine. And an attached EXE program may be absolutely anything. However, floppy disks and other means of moving data from one computer to another are also frequent sources and carriers of infection. Inserting a floppy in an infected computer can infect the floppy, and inserting the infected floppy in an uninfected computer can and probably will infect that computer. The most important thing you can do is to install a reputable anti-virus package and keep it up to date.