First, I don't believe the Always Ignore has anything to do with Searching. That particular list is used when you instruct RoboHelp to build an index using the Smart Index Wizard.
Actually, something like "Office Not Installed" is a perfect use of the Index. But I'm guessing here that your system doesn't include an index in favor of trawling through the pile of words in the project.
If you aren't using an index yet, I'd highly suggest you investigate it. In a properly indexed system your users will find what they are looking for infinitely faster and easier than they ever will by using Search.
Just a thought... Rick
Helpful and Handy Links
Indexing is something we are looking at for the future, but we have 14,000 topics in our Knowledge Base and a userbase that is very used to searching in the way they have been for the past five years, and so now we have this particular, specific problem of using the word "not" in a search term and getting that grammatically terrible error message, "The words you typed is not a valid expression."
I see that this message has never been replaced in RH, and simply want to update the date and version for the problem.
Version RH11. I'm still getting the odd "The words you typed is not a valid function."