For the third year in a row, Apple (AAPL) has come in first in an annual survey of computer reliability conducted by Rescuecom, a national tech support company based in Syracuse, N.Y.

Rescuecom measures reliability by dividing each manufacturer’s U.S. market share, as measured by IDC, by the percentage of calls requesting Rescuecom’s service on each maker’s machines.

Although Apple had the highest score in Rescuecom’s 2010 report, its lead over its nearest competitor shrank from more than 200 points in 2008 to just 60 points this year.

In a press release issued Monday, Rescuecom president Josh Kaplan singled out Apple, Asus and, in a negative way, Dell (DELL):

“Apple finished the year with a higher score than Asus,” he wrote. “However it was Asus who won the spotlight with its impressive 190.7% growth, year over year.

“Conversely, Dell’s negative growth and poor reliability scores led to an increased need for Dell customers to seek computer repair. This performance stopped Dell from reaching a top five position.”

In its 2006 report, Rescuecom indicated that its survey was based on 20,000 calls to its 800 number. No comparable figure was provided in this year’s report.

And as several readers have pointed out, Rescuecom’s numbers don’t include machines repaired either by corporate IT departments or through manufacturer-supplied service contracts, such as AppleCare.

[Follow Philip Elmer-DeWitt on Twitter @philiped]