One of the most common ways in which a computer becomes virus-infected is that its user actively causes the virus to run, not realizing that the file being opened could contain a virus. This can happen when, for example, you double-click a virus-infected attachment to an e-mail message.
Viruses are sometimes compressed inside Zip files, which are then sent as e-mail attachments. When you receive a Zip file as an e-mail attachment, double-clicking the attachment will open the Zip file in WinZip (or whatever Zip utility is installed on your system). If, within your Zip utility, you then double-click the virus-infected file, the virus can run and your computer can become infected. (An up-to-date virus scanner, scanning files in real time, will stop most infections, but the newest viruses may escape detection.)
One reason that viruses are able to spread is that double-clicking a file has historically been a safe thing to do. It is easy to casually double-click a file received as an e-mail attachment, or a file within a Zip file received as an e-mail attachment.
Unfortunately, because of the recent proliferation of viruses, it is no longer safe to routinely double-click files, especially files received as attachments to suspicious e-mail messages or downloaded from untrusted web sites. This issue has become so important that recent versions of Internet Explorer and of e-mail programs such as Outlook, Outlook Express, and Windows Mail warn you when you are about to work with potentially unsafe file types.
Because Zip files can sometimes contain virus-infected files, this version of WinZip displays similar warnings in certain situations when a Zip file that you are working with contains files of potentially unsafe types.
About Potentially Unsafe File Types
The file type of a file, sometimes referred to as its extension, is the last portion of the name of the file. For example, if you have a file called "memo.doc", the last portion of the name of the file (that is, the portion following the final period) is "DOC". This file therefore has a file type of DOC, and would sometimes be referred to as a DOC file or a .DOC file.
Windows generally uses the file type to decide how the file will be handled when you double-click the file's icon. For example, DOC files most often contain Microsoft Word documents; if you double-click a DOC file and Microsoft Word is installed on your system, Windows will open the file in Microsoft Word.
As a second example, the file "game.exe" has a file type of EXE. This type of file normally contains a program, and if you double-click it, Windows will run the program.
The EXE file type is an example of a potentially unsafe file type. While the vast majority of EXE files that you are likely to encounter are safe and useful, malicious individuals sometimes write programs that can harm your computer and distribute them in the form of EXE files.
There are a number of other file types that are often considered to be potentially unsafe because they have been used to spread viruses. There is no definitive list of these file types that everyone would agree with, and to some extent the list is constantly changing, but here is a short list that is being used:
ADE, ADP, APP, ASA, ASP, BAS, BAT, CER, CHM, CMD, COM, CPL, CRT, CSH, DLL, EXE, FXP, HLP, HTA, HTM, HTML, HTR, INF, INS, ISP, ITS, JS, JSE, KSH, LNK, MAD, MAF, MAG, MAM, MAQ, MAR, MAS, MAT, MAU, MAV, MAW, MDA, MDB, MDE, MDT, MDW, MDZ, MHT, MHTM, MHTML, MSC, MSI, MSP, MST, OCX, OPS, PCD, PIF, PRF, PRG, REG, SCF, SCR, SCT, SHB, SHS, TMP, URL, VB, VBE, VBS, VBX, VSMACROS, VSS, VST, VSW, WS, WSC, WSF, WSH, XSL
All of these file types have legitimate uses under appropriate circumstances. The simple fact that a file is one of the types listed above does not mean that there is something "wrong" with the file or that it is infected with a virus. But if you receive a file with one of these types from an untrusted source or in unexpected or suspicious circumstances, you should not work with the file until you are sure that it is safe.
Of course, as mentioned above, there is no definitive list of which file types are safe or unsafe, so even with file types that are not on the above list, you should use appropriate cautions.