If you are the administrator of a business organization with a network of many computers, I wish you luck!
If it is a personal computer, you may want to check/try:
If you have recently installed anti-virus software, check the configurations. Perhaps you have inadvertently checked a restrictive e-mail scanning/usage option.
If that does not work, try disabling/suspending the anti-virus software to see if it works. If it works, call customer support of the anti-virus software.
2. Windows 2000
If you are running windows 2000, and this is the first time the problem appears, it may be caused by the very restrictive default security options of the operating system.
3. XP, etc.
Check what recent changes to settings you may have made regarding security. This may give a clue as to where the problem may be.
In questions of this nature, it always helps to know if e-mail attachment has ever worked before, when does it stop working, and what recent changes to settings you have made.
Also, the type of file you are trying to open may have an effect on the permissions. Most anti-virus software would not let you open files with extensions .exe, .dll, etc. for fear of contamination by viruses.