Just three days were enough to push computer sales for the week up 40%

The sharp spike in the chart at right is the Windows 7 effect PC makers have been waiting for.

In a note to clients issued Monday afternoon, Morgan Stanley’s Kathryn Huberty reports that NPD data for the week ending Oct. 24 — which included three days of Windows 7 sales — show PC sales jumping 40% year over year.

This was particularly encouraging, she writes, because sales in the early part of the week likely reflected the same pre-Windows 7 declines as the previous two weeks. PC buying for the weeks of Oct. 17 and Oct. 10 was down 29% and 2%, respectively, as consumers waited for Microsoft’s (MSFT) new operating system to launch.

Click to enlarge.

Huberty dismisses concerns that computer vendors over-shipped ahead of the Windows 7 launch, demonstrating in a second chart that PC inventory levels are still below average.

So which computer maker’s shares are likely to benefit most? In a separate note issued Tuesday, Huberty singles out Hewlett-Packard (HPQ), whose potential for growth she says is under-appreciated by investors.

She also likes Apple (AAPL) as an investment, but less for its computers than for the new distribution agreements that are driving iPhone sales.