More 'scientific' than humorous, but hey... I found it pretty entertaining.
Bill Gates is so rich most of us can't even imagine how rich he really is. This page should give you an idea...

Microsoft claims to have sold approximately 120 Million copies of Windows 95. If every one of those copies crashes or otherwise requires a reboot an average of once per day, and Bill were charged $1 per crash or reboot, his money would last 129.23 days or 0.35 years.
Bill could give $2.47 to every person on the entire planet! And he'd still have $56,440,511 left over for incidentals....Bill could give $53.42 to every person in the United States.
You could cover the entire surface area of Vatican City with Bill's dollar bills 356.89 times. That would be to a depth of 1.53 inches.
A Boeing 747-400 holds 27,467 cubic feet of cargo. This means that Bill's money in $1 bills would fill 22.94 Boeing 747-400 cargo compartments!

However, 27,467 cubic feet of $1 bills or 675,850,255 (675.85 million) bills weighs 1,379,286 pounds or 689.64 tons. This is quite a bit more than the 124 ton rated carrying capacity of a Boeing 747-400.

So, if Bill actually wanted to fly all his money someplace (say to Manhattan, to carpet it), he'd need 127.6 Boeing 747-400s.
He could buy 51.7 million copies of WinXP Pro.
Gah.