After three major viruses exploited flaws in Windows operating systems within a short time period in 2003 , Microsoft changed its development priorities , putting some of Longhorn 's major development work on hold while they made new service packs for Windows XP and Windows Server 2003 . After three major viruses exploited flaws in Windows operating systems within a short time period in 2003 , Microsoft changed its development priorities , putting some of Longhorn 's major development work on hold while developing new service packs for Windows XP and Windows Server 2003 .