What kind of pop ups are they? I am using Word 2016 on both of my Macs (also running High Sierra, OS 10.13.2), and I am not getting any pop ups (in fact, that's true for any app I use, ie, Excel 2016, Outlook 2016, Quicken 2017, etc.).
And do the pop ups only happen with Word 2008? What about other apps, like Excel 2008 (if you use it)? It would be strange if this was confined to only 1 app, unless you downloaded/"imported" a Word document from somewhere (or maybe via an EMail), and it was somehow "infected".