Enable file extension viewing
The invisible extension - what you can't see, can hurt you. By default, Windows does not have file extension viewing enabled. This allows virus writers to distribute executable files disguised as something non-executable. For example, an .EXE file might appear to be an innocuous text file.

Enabling file extension viewing in Windows 95/98/NT
In Windows 95/98/NT, enable file extension viewing by opening Windows Explorer. Click View | Options | View and uncheck the box for "Hide file extensions for known file types". You can also do this by via Windows Explorer View | Options | File Types menu. Locate the desired file type(s) and check the "Always Show..." checkbox).

Enabling file extension viewing in Windows 2000 and XP
In Windows 2000 or XP, open Windows Explorer and choose Tools | Folder Options | View or Tools | Folder Options | File Types, locate the file type(s) desired and choose Advanced.

Then check the box "Always Show Extension".

Enabling file extension viewing for .SHS files
The above instructions will display all file extensions except for .SHS files. To display .SHS file extensions, one additional step is required. After following the above instructions, users must then edit the Registry, HKEY_CLASSES_ROOT\ShellScrap, deleting the value "NeverShowExt".

Executable file extensions
Following is a partial list of file types that should be considered suspicious when received in email and should not be opened unless you requested or expected the attachment:

ADE - Microsoft Access Project Extension
ADP - Microsoft Access Project
BAS - Visual Basic Class Module
BAT - Batch File
CHM - Compiled HTML Help File
CMD - Windows NT Command Script
COM - MS-DOS Application
CPL - Control Panel Extension
CRT - Security Certificate
DLL - Dynamic Link Library
DO* - Word Documents and Templates
EXE - Application
HLP - Windows Help File
HTA - HTML Applications
INF - Setup Information File
INS - Internet Communication Settings
ISP - Internet Communication Settings
JS - JScript File
JSE - JScript Encoded Script File
LNK - Shortcut
MDB - Microsoft Access Application
MDE - Microsoft Access MDE Database
MSC - Microsoft Common Console Document
MSI - Windows Installer Package
MSP - Windows Installer Patch
MST - Visual Test Source File
OCX - ActiveX Objects
PCD - Photo CD Image
PIF - Shortcut to MS-DOS Program
POT - PowerPoint Templates
PPT - PowerPoint Files
REG - Registration Entries
SCR - Screen Saver
SCT - Windows Script Component
SHB - Document Shortcut File
SHS - Shell Scrap Object
SYS - System Config/Driver
URL - Internet Shortcut (Uniform Resource Locator)
VB - VBScript File
VBE - VBScript Encoded Script File
VBS - VBScript Script File
WSC - Windows Script Component
WSF - Windows Script File
WSH - Windows Scripting Host Settings File
XL* - Excel Files and Templates