I suspect you have a problem with the mechanism that sends the file to its default application. I had similar problems, even on Windows XP.

If it is a Word file (for example), try to open the file with Word, with the "File | Open" command - I suspect you will not get the error this way.

>Here's one I've never seen before. This is Win98SE. There is a legacy DOS based app that runs off the network on these machines. Netware server for this particular program.
>
>I've always used the icon file that sits on the server. Never been an issue, until these two machines I just obtained. I did the OS install. The machines are do not have identical hardware. Double clicking on the icon gives the following message, in a white box:
>
>
>SYSTEM ERROR
>"The file you are trying to open is already in use by another program. Quit the other program and then try again."
>
>Pressing "CANCEL" allows the program to go ahead and run.
>
>Copied icon to local hard drive, and pointed to it. No error any more.
>
>Any thoughts on why this happened?
>
>Hmmm...I wonder if one of the recent Microsoft patches is the culprit.