From FCKeditor Docs

Aspell and Windows 2003 Server Configuration

These are some tips when configuring Aspell to run in a Windows 2003 Server using the PHP spell checker:

• SpellerPages calls the aspell.exe executable, so you must have "execute" and "write" permissions over the Aspell installation directory (usually C:\Program Files\Aspell).
• The aspell.exe executable is called by using the shell_exec command, which directly calls C:\Windows\System32\cmd.exe, so you must have "execute" permission over it (IUSR_xxx user).
• You must have "read" and "write" permissions in the temporary folder (usually C:\Windows\Temp). Attention, this time you need to give the permissions to the IIS_WPG user.