File extensions exe : The EXE executable (program) file

An executable file is a program that can be executed on the various Microsoft operating systems (Dos,windows 98/xp/vista).

What is rundll32.exe?

Rundll32.exe is a process that allows dynamic link libraries (DLLs) to be executed.

Default folder:
C:\Windows\System32

In the above screenshot, rundll32.exe is running as the current user (Mike). Due to the nature of this process, it can run as any user.

It is common for virus and spyware to disguise themself as the genuine one.
The following malware is known to disguise itself as rundll32.exe:

W32.Miroot.Worm (%SystemRoot%)
Backdoor.Lastdoor (%SystemRoot%System32)
Troj/AnaFTP-01 (%SystemRoot%undll.exe)
W32.Rbot-GSJ (%SystemRoot%undll.exe)
W32.Agobot.EQ (%SystemRoot%System32%undll.exe)

Common Errors

Windows cannot find C:\Windows\Rundll32.exe make sure you typed the name correctly and try again.
This issue occurs because of a missing or corrupted Rundll32.exe file. This issue may also be caused by a virus.
Solution:
1. Insert the Windows XP CD ([Cd-Driver])
2. Click Start, and then click Run.
3. Type expand [Cd-Driver]:\i386\rundll32.ex_ c:\windows\system32\rundll32.exe in the Open box
4. Restart the computer.

Rundll32.exe at 100% CPU Usage
Because rundll32.exe allows any dll to be executed, and as such the executing dll will appear as rundll32.exe, any misbehaved dll could cause rundll32.exe to use 100% CPU time.

Product name:  Microsoft(R) Windows (R) 2000

Description: Modulo di esecuzione DLL come applicazioni

Manufacturer:Microsoft Corporation