XP has been top of the pack for a long time now, but it looks as though Windows 7 may be poised to overtake for the top spot. Net Applications took a snapshot of the operating system landscape in May and found that the gap between Windows XP and Windows 7 was the narrowest it has ever been in the history of the two OSs.
The study placed XP's market share at 44.8% and Windows 7's market share at 40.5%. Soon, XP support will be dropped by Microsoft and businesses will be forced to upgrade. Considering some of the possible issues with Windows 8, it's likely they will jump to Windows 7 which will further push it into the top spot.
This fact has been inevitable. Microsoft itself has been pushing the upgrade from XP to 7 by telling companies explicitly not to wait for Windows 8. Even still, OS migration is not an easy task which explains the slow uptake. After all, XP is a stable, proven, and robust operating system so there has been no reason to upgrade.