IDC’s report on plummeting PC shipments has hit Microsoft (MSFT) hard and now Goldman Sachs analyst Heather Bellini has downgraded the company from Hold to Sell while predicting that its market share will “gradually deteriorate” unless it makes bold changes to turn itself around. Per Bloomberg, Bellini writes that Microsoft “faces critical secular challenges given the deteriorating PC demand backdrop” and that its market share will “gradually deteriorate unless Microsoft successfully repositions itself as a more meaningful participant in the new era of consumer compute” that includes tablets and smartphones. Bellini’s note is very similar to a note released Wednesday by BGC analyst Colin Gillis, who said that Microsoft’s “minimal participation” in the mobile market represented “an increasingly urgent problem that may prove difficult to reverse.”