http://support.microsoft.com/kb/555067
If that doesn't fix it, using Notepad or any text editor, create a .REG file as shown below. Name it EXEFIX.REG, or whatever, doesn't matter so long as it ends in .REG:
Windows Registry Editor Version 5.00[HKEY_CLASSES_ROOT\.exe]@="exefile""Content Type"="application/x-msdownload"[HKEY_CLASSES_ROOT\.exe\PersistentHandler]@="{098f2470-bae0-11cd-b579-08002b30bfeb}"
Windows Registry Editor Version 5.00
[HKEY_CLASSES_ROOT\.exe]@="exefile""Content Type"="application/x-msdownload"
[HKEY_CLASSES_ROOT\.exe\PersistentHandler]@="{098f2470-bae0-11cd-b579-08002b30bfeb}"
Then double-click on it and answer any prompts that come up.