U.S. demand for personal computers slowed in September, sales tracker NPD reported, with Apple’s showing the second straight month of deceleration in unit growth.

NPD reported that Apple, which is scheduled to report its fourth quarter earnings Tuesday afternoon, posted an 11.4 percent year-over-year increase in revenue and a 14.5 percent hike in units sold.

That compared to a 19.5 percent increase in PC revenue in August, a 38.2 percent increase in July, and a 21.2 percent increase in September 2007. Apple’s PC unit increases registered 22.4 percent in the year-ago month and 42.9 percent in July 2008.

Shares of Apple fell $4.23, or 4.3 percent, to $94.21 in afternoon trading Tuesday.

The consensus estimate of 29 analysts polled by Thomson Reuters for Apple’s fourth-quarter earnings per share was $1.11 versus $1.01 in the year-ago quarter.  The mean revenue estimate for the quarter ended September 30 was $8.045 billion.

But Wall Street’s focus is expected to be on Apple’s guidance for the December quarter and all of fiscal 2009. In a research note earlier this month, Credit Suisse  analyst Bill Shope cut his December quarter revenue estimate for Apple from $11.47 billion to $9.78 billion and his earnings per share estimate from $1.82 to $1.51.

Mr. Shope slashed his forecast for fiscal 2009 overall, targeting revenue of $35.4 billion and $5.19 in earnings per share, down from $42.11 billion and $6.41 respectively.

His price target went from $200 per share to $135. Credit Suisse makes a market in Apple securities.

NPD also reported that Hewlett-Packard’s personal computer revenue fell 3.9 percent and units sold declined 3 percent versus the 2007 period.

NPD tracks U.S. retail, distributor, and business reseller channels.