Dell has enjoyed the top spot for a long time, but many are fighting back. HP has always been close, but just recently they have surpassed Dell in sales. For desktop PCs in the last quarter, with 9,831,000 PCs shipped, they passed Dell, which had a total of 9,803,000. A very tiny fraction, one that many, including the IDC, consider more of a tie than anything:

Gartner pointed out that it is the first time since the fourth quarter of 2003 that HP has taken the top rank in worldwide PC shipments. However, IDC said it considers it "a statistical tie" between the two vendors.
Dell has been hit hard by some PR issues, but likely will now want to come back in full force. HP and Dell together compromise roughly a third of PC shipments worldwide, so if they get into a price or product war it will be apparent to just about every market.