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Bill Gates Net Worth Page 
Based on Microsoft's current stock
price,

No such file or directory: Cannot open BGquotes.txt 

We obviously do not have a full reckoning of everything Bill owns. But, according to the Yahoo! Insider Trades site, Bill has shares of MSFT.
This figure reflects all trades made through , which is the most recent trade. (Thanks to Jeff Call for the pointer!)
As of ET on , Microsoft was selling for $.
Some simple multiplication tells us that he owns $0.00 worth of Microsoft stock. (That's $0.00.)
Microsoft has a total of 10,700,000,000 shares outstanding, worth a total of $0, which is Microsoft's market capitalization. (That's $0.)
Bill's shares represents 0.00% of Microsoft.
On March 13, 1986, the day Microsoft went public, according to Microsoft's Public Relations Department, Bill owned 11,142,000 shares of MSFT stock. At MSFT's IPO price of $21 per share Bill's initial fortune was worth $233,982,000 or $233.98 Million. Adjusted for splits, if Bill had not sold any stock since then, he would have 3,208,896,000 shares. Those shares would be worth $0.00 or $0 today. Thanks to Michael Noer and Julie Sullivan at Wired Magazine for their help here.
Microsoft stock's alltime high price (adjusted for splits) is $. That means that Bill's alltime high net worth (if he hadn't sold any shares) is: $0.00 or $0. Microsoft stock is currently AT its alltime high!
If Bill had never sold any of his MSFT stock, at his maximum price, Bill's share would be worth: $0.00 or $0.
Microsoft went public on March 13, 1986. Its IPO price was $21/share, which, after adjustments for splits is equivalent to 7.29¢.
Microsoft stock has split a total of 9 times, increasing the number of shares, and decreasing each share's value by a factor of 288. If MSFT stock had never split, the share price today would be $0.00. (Thanks to Michael Chauvin for the suggestion)
The day Microsoft went public, Bill's fortune was worth $233,982,000 or $233.98 Million.
In the time since March 13, 1986, Bill's Net Worth has increased by $233,982,000.00 (that's $233982000.00). Let's look at how much Bill has made for each of various periods of time...
Unit of time  Quantity since March 13, 1986  Dollars made during each of these periods 

Seconds  1,042,122,980 (1.04 Billion)  $0.22 
4 Seconds  260,530,745.00 (260.53 Million)  $0.89 
Minutes  17,368,716.33 (17.37 Million)  $13.47 
Hours  289,478.60 (289.5 Thousand)  $808.28 
Days  12,061.60 (12.1 Thousand)  $19,398.90 ($19398.9051081092) 
Weeks  1,723.08  $135,792.33 ($135792.335756765) 
Months  402.05  $581,967 ($581967.153243277) 
Years  33.04  $7,080,600.36 ($7080600.36445986) 
Thanks to Robert Tweardy and Frank Isaacs for their information!
Note: We give 4 seconds its own entry because that is the period of time that Brad Templeton of the Bill Gates Wealth Page says it takes to pick up found money from the ground. Brad says that this amount is not worth Bill stooping to pick up. Draw your own conclusions.
Imagine an endless line of $1 bills laid end to end. If you began traveling along this line picking up dollar bills March 13, 1986 (the day Microsoft went public), and you wished to accumulate wealth at the same rate Bill has since that date, you would need to travel that line of bills picking them up at 0.07 MPH or 0.12 KPH. 
Since March 13, 1986, Bill has been making money at a compounded interest rate of: 100.00% per year. Compare that to the historic growth rate of the Dow Jones Industrial Average, which is about 11%.