In a report to clients issued Monday afternoon, Piper Jaffray’s Gene Munster — a long-time Apple (AAPL) booster — found much to cheer about in the NPD Group’s U.S. retail sales data for January and February.

“We are buyers of AAPL based on Feb. NPD data,” he writes. He points to two trends in particular:

• Mac unit sales are up. An average of 39% year over year for the first two months of the March quarter, which according to Munster translates into sales of somewhere between 2.8 and 2.9 million Macs for the full quarter. The Street, he says, is looking for Mac sales to be up only about 22%.
• iPod unit sales are up. After a series of 16 consecutive monthly declines, iPod sales are up 7% year over year for the last two months, suggesting total iPod sales of 9 to 10 million for the March quarter. The consensus is closer to 9 million.

Munster is quick to point out that these year-over-year comparisons are easier this quarter — and will be for the first half of 2010 — given the declines both products suffered during the recession last year (see spreadsheets below the fold). There’s also some sobering news in the ASP (average selling price) of both product lines. The Mac’s ASP is down 10% versus the 7% drop Munster had built into his model. ASPs for iPods were up slightly (3%), perhaps due to a higher mix of iPod touches. Munster, however, was expecting a 15% increase.

Below the fold: The raw NPD data.

[Follow Philip Elmer-DeWitt on Twitter @philiped]