Introduction - If you have any usage issues, please Google them yourself
Virus writing is not as hard as you might first imagine. To write an effective virus, however, you*must* know assembly language. Short, compact code are hallmarks of assembly language and these are desirable characteristics of virus. However, it is*not* necessary to write in pure assembly. C may also be used, as it allows almost total control of the system while generating relatively compact code (if you stay away from the library functions). However, you still must access the interrupts, so assembly knowledge is still required. However, it is still best to stick with pure assembly, since most operations are more easily coded in assembly. If you do not know assembly, I would recommend picking up a copy of The Microsoft Macro Assembler Bible (Nabajyoti Barkakati, ISBN#: 0-672-22659-6). It is an easy-to-follow book covering assembly in great detail. Also get yourself a copy of Undocumented DOS (Schulman, et al, ISBN#0-201-57064-5), as it is very helpful.