But most companies have work to do, Williams said, citing the 10% threshold of Windows XP SP2 systems. "It's unrealistic to expect them to execute a deployment of in the next three weeks," he said. "But they should determine who is affected and get them updated to Windows XP SP3 immediately."

Windows XP SP3, which , is available as a free upgrade to all Windows XP users. Microsoft has promised to support XP SP3 with security updates until April 8, 2014.

Softchoice's data is similar to numbers produced last month by Qualys, which said that approximately running some version of XP were using SP2.

According to Web metrics company Net Applications, 62.5% of all personal computers worldwide ran Windows XP in May. Net Applications has tracked an 11-point drop in XP's usage share in the last 12 months.

Williams expects that number to fall even faster with Windows XP SP2's retirement. "This represents the death knell of XP," he said. "[Windows XP SP3] is only a stay of execution."