Mathematicians at UCLA have discovered a 13-million-digit prime number, a long-sought milestone that makes them eligible for a $100,000 prize.

The group found the 46th known Mersenne prime last month on a network of 75 computers running Windows XP. The number was verified by a different computer system running a different algorithm.

"We're delighted," said UCLA's Edson Smith, the leader of the effort. "Now we're looking for the next one, despite the odds."

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See the gory details @ http://www.math.ucla.edu/~edson/prime/ like the following question and answer:

Q.  When did the discovery occur?  What kind of computer was used?

A.  The UCLA Mersenne Prime was reported on August 23, 2008 on a computer named zeppelin.pic.ucla.edu, a Dell Optiplex 745 running Windows XP with an Intel Core 2 Duo E6600 CPU running at 2.4 GHz.  The name "zeppelin" was part of our Classic Rock Band series of computers.

http://blogs.technet.com/keithcombs/archive/2008/09/27/ucla-group-discovers-massive-prime-number-with-windows-xp.aspx