WinPcap used to provide a silent installer, but they took it out of the open source installer due to misuse. The official silent installer is still available via:

• WinPcap Pro, $4000
• The Network Toolkit (portable Wireshark), $40

You can get a third-party silent installer from:

• Download the Windows version of nmap and extract winpcap-nmap-4.02.exe. When you want to install WinPcap, run it with "/S". (it's a custom WinPcap installer, created because the official one "phones home")

You can do a silent install yourself:

• extract the following files using 7-zip:
  • $SYSDIR\drivers\ contains multiple copies of npf.sys. Extract all of them into a new folder using 7-zip's "Auto Rename", open the Properties dialog for each, and look at the Description:

    "npf.sys (NT4) Kernel Driver"	NT v4.x
    "npf.sys (NT5/6 x86) Kernel Driver"	2000/XP/Vista/..., Intel
    "npf.sys (NT5/6 AMD64) Kernel Driver"	2000/XP/Vista/..., AMD
    ?	95/98/ME (available in earlier versions)

  • $SYSDIR\ contains multiple copies of packet.dll. Same deal:

    "packet.dll (NT4) Dynamic Link Library"	NT v4
    "packet.dll (NT5) Dynamic Link Library"	2000/XP
    "packet.dll (Vista) Dynamic Link Library"	Vista

  • extract all other DLLs from $SYSDIR\ (eg. pthreadVC.dll, wpcap.dll, WanPacket.dll)
• to install:
  • copy the appropriate npf.sys to c:\windows\system32\drivers\ (unfortunately, this step always needs to be done, even for portable apps)
  • if you're using a portable app, just make sure all WinPcap .DLLs are in the same directory as your app; to install permanently, copy all .DLLs to c:\windows\system32\

Yes, Wireshark does offer a portable version. However, its installer isn't silent: