This is the code I'm running:
RunWait("ComSpec & " /c Start 'D:\Program Files (x86)\Pidgin\pidgin.exe'")
I have also tried without semiquotes and with quotes but then I get syntax errors.
The problem is that the file path on your computer has spaces in it and needs double-quotations around it.
Try "D:\Program Files (x86)\Pidgin\pidgin.exe" from the command prompt with the double-quotations and make sure the program starts.
Once you know it works you can add it to your AutoIt code like so (notice that it is surrounded by single-quotations).
RunWait(@ComSpec & " /c " & '"D:\Program Files (x86)\Pidgin\pidgin.exe"')