In The slow road to Windows XP (CNet) says:
Windows XP use surged to 38 percent by the first quarter of this year, up from 6.6 percent in the third quarter of 2003. However, the popularity of Windows 2000 has remained high, with the venerable operating system still in use in 48 percent of business PCs during the first quarter of 2005, down just four percentage points from the third quarter of 2003.
(later…) By not releasing a more full-featured service pack [for Windows 2000], Microsoft may be trying to send a signal that customers need to upgrade to get new features.
Over half are still using Windows 2000. To get new features they need to upgrade.
Tell me again how Internet Explorer is free? It’s tied to the operating system and won’t be released, and just about everyone indicates that Microsoft uses this technique to force people to upgrade.