What many want to know

• Why are there files with ending with an underscore _?
• Programs which worked before show an "internal error"
• How an application is started if a document is clicked on

Files ending with ex_, dl_, ...

Most people have already seen files (most often during installation) those last letter is replaced by an underscore. Maybe you already tried to rename it to *.exe or *.dll in order to get them to run, however without success. But the idea was not that bad: These files contain really *.exe or DLL files. But these files have been compressed. Similar to WinZip or LHA Microsoft offers a file compressor for shrinking files: COMPRESS.EXE. It creates these files. They are uncompressed with functions residing inside the windows system file LZEXPAND.DLL.
Without much effort, programmers are able of shrinking their files. There is no need to write an own compressor.

Occurance of unknown errors

A problem has been running OK for some time, but now it shows errors like:

Internal Error

or:

system error

Some also exit on their own.
These problem is often created by DLLs containing program code. During the installation of new programs they are replaced by outdated or incorrect versions. Especially files like comdlg32.dll, which are rewritten very often, cause this. Quite often testing or beta versions of the DLL got into the public. Fix: Get another version of it and replace the current one
Hint: In the Windows directory there is a hidden folder called sysbackup, often containing the old version.

How a document is linked with an application

The mechanism allowing to start a file within its according application just by clicking on it is quite simple: First, the OS looks into the registry if the file ending has an entry in it. If not, the user is queried for a suitable application. Otherwise, the content of the entry is executed: Either the file is started on its own (that means it is an executable file), or the entry names a program to which the file is transferred using a command line parameter.
Note: Especially windows programs are heavily using command line parameters. The old UNIX technique is up to date as it has never been before. More info about it can be found in the resource kit on the windows installation CD.