Petite is a Win32 (Windows 95/98/2000/NT/XP) executable (EXE/DLL/etc...) compressor. The compressed executables decompress themselves at run time, and can be used just like the original non-compressed versions. Petite also adds virus detection to the compressed executables - they will check themselves for infection every time they are executed.

Site: http://www.un4seen.com/petite/