Microsoft Corp.’s (Nasdaq: MSFT) Windows XP and Windows 2000 operating systems have been steadily losing ground during the first half of 2007 as their newer cousin, Windows Vista continues to gain market share, new data shows . Mac OS remained steady.

Vista - which was released at the end of January of this year – is being used on 4.52 percent of tracked computers, according to web tools maker Net Applications. When it was first released, Vista had a 0.18 market share.

Vista's market gains have come at the expense of the older Windows operating systems, which have shown a steady month-to-month decline. In January, XP and 2000 had shares of 85.02 percent and 4.93 percent, respectively. By June, however, XP had declined to 81.94 percent of the market while Windows 2000 had a share of 4 percent.

Apple, Inc.’s Mac OS experienced a relatively steady market share since January when it reached 6.22 percent. In June, systems with Mac OS installed were 6.0 percent.

Net applications says data is aggregated monthly and is available to subscribers for the prior month shortly after the current month begins.