Hi
If I open the Properties on my c: drive, then I can see that the 'Index drive' checkbox is checked, and Windows search is running, however the search is SLOW and I get a messages saying that the indexing service is not running
Directory Opus Pro 10.5.4.0 (5080) x64
OS 6.3 (B:9600 P:2 T:1) SP 0.0
This doesn't really have anything to do with Opus. Both the Properties dialog and the Indexer are part of Windows.
Maybe check in your Services control panel that the Windows Search service hasn't been disabled.
Hi
Sorry I do not really understand your answer 
My 'point' is that the search is SLOW (Running on a SSD disk) and then Opus says that it is due to the indexing service not running..
AFIK indexing service in Windows 8.1 is Windows Search and that service is running, and the 'Index this drive' property is checked, so there should not be any reason for Opus to be slow nor say that the indexing service is not running... Or do I miss something?
Go to Control Panel, Indexing Options and make sure the folder is included in the list there.
Here's a guide from a magazine that includes what to do with screenshots:
blog.laptopmag.com/windows-81-search-index
Or here's a YouTube video that looks like it covers the same sort of thing:
Adding to Leo's answer, sensible indexing software will not index files and folders that it makes little sense to index.
The point of Windows Search is to find files that you want to read and work on. PDF files, for example. So by default most index and search software will exclude, for example, c:\windows and further on down, also [b]\program[/b] folders. So when you do want to find something down there, there is no index for Windows Search, or Opus, to work from. It has to search "on the fly".
Indexing anything and everything slows down the more normal index and search operations and requires a much larger disk to hold a bigger index.