Most programs that have an installation routine tell me it is highly recommended that I close all other programs before installing.
Why is this? What’s the danger? Why is there this danger? Etc.
-FrL-
Programs sometimes share components & libraries. If one is in use, your program may not install properly.
It’s rather pointless these days on Windows XP and Vista. The rationale, as was noted by ASAKMOTSD, is that other programs may be using files that the program you are installing needs to update. However, the reality is that the operating system’s shell (Windows Explorer) is using many of those same libraries and you can’t easily shut it down.
What really happens now is one of two things - either the replacement of the file gets pended until you reboot (which is why some installations tell you that you must reboot before using the application) or the shared components aren’t really shared at all and you just install new copies of the files that are used only by the application you’re installing. The latter is called Side-By-Side DLLs or Private DLLs. This was the solution to “DLL Hell” which was the result of having applications share DLLs - you eventually ended up with broken applications when one of them updated a shared DLL and an older program didn’t work with the newer DLL. Now an application can specify that it requires version 6.0.0.1863 of comctl32.dll, and it will always have that version even if another program installs a newer version.
Short version - don’t bother closing applications unless the installer tells you the particular ones that need to be closed. Otherwise, it’s just a vestigial blanket statement.