It could be a case when you may find a file in Windows 7 or Vista which you cannot open, and this may happen with a file type which was used to open properly before but suddenly it is shown as unrecognized file which cannot open until you assign a program to open it.
This type of problem generally happens when you PC got infected with some malware or spyware etc which might have caused this file association problem in Windows. Here is how you can fix this problem of file association in Windows 7 and Vista with a free tool.
This Free tool is called File Association Fixer which detects and fixes the file association problem in windows 7 and vista. In order to fix the association for a particular file type simply click on the file type association icon or name and then you might need to reboot.
This tool currently offers 18 file association fixes for Windows 7 & 26 fixes for Windows Vista, we would always recommend to use system restore to restore your computer before using such tools, you can use some quick system restore tools like Quick Restore Maker and Single Click System Restore application.