What is it?
win32s.exe is a file associated with the w32.mydoom.v@mm worm.

What does it do?
W32.Mydoom.V@mm is a mass-mailing worm that downloads an excutable file.

When W32.Mydoom.V@mm is executed, it does the following:

• Adds the value:
  
  "Win32System" = "%WinSysDir%win32s.exe"
  
  to the following registry key:
  
  HKEY_CURRENT_USERSoftwareMicrosoftWindowsCurrentVersionRun
  
  so that it is executed every time Windows starts.
• Creates a mutex named "LLLf54fxrDLLL" so that only one copy of the worm is run on the infected computer.
• Retrieves the email addresses from the Windows address book files.
• Retrieves the email addresses from the files that have the following extensions on drives C through Y:.........
• ........

More info and removal instructions
@symantec