OK, here's the thing, please don't fucking do this.

A Denial of Service attack is probably the easiest way to cost big businesses that rely on web sites lots of money. In short, they work by overwhelming a web server with requests. Currently, there's very little a company can do, short of trying to catch an attack before it happens. On the other hand, I'm not going to get into how to hide your tracks, so if you do this, you're probably screwed.

  1. Write a trojan horse, or find one that suits your needs. Most likely, you'll have to write your own, to avoid having the trojan detected by anti-virus programs. The trojan horse should check to see if the computer is online and if it is the right date, start bombarding the chosen victim's servers with repeated requests for web pages. Be creative! Mix up which pages are requested, and what servers are attacked. Bonus points for insulting uses of the server's graphical 404 message, or requesting obscene or politically-named web pages.
  2. Collect unsecured shares on windows machines. These are the shares set up under MS Networking (file sharing) to share on local networks, but are also vulnerable to attacks from the internet at large. You will need a high-speed internet connection, such as a cable modem or a DSL link. Get a decent UDP port scanner, and aim it at machines in your own network. If you're in a busy area, you can easily find a few dozen in little time at all. Unfortunately, for this attack, we'll need lots and lots of machines, so you may need to expand your search into neighboring subnets. The kinds of shares we're looking for are set up by morons, who allow access to certain directories with no password. Fortunately, there's no shortage of morons on the internet. To actually browse a computer's shares, all you really have to do (on a windows machine) is to use the UNC notation; that is, run \\machine.ip.address or \\machine.ip.address\share, if you have that.
  3. Install the trojan horse. We need to copy the trojan horse to a well-hidden location and cause the machine to run every time it starts windows. If you have access the windows directory, you can either edit win.ini to include the line "load=trojan.exe" or edit the registry (not an easy process, as it isn't plaintext). There are a couple places in the registry where you can enter a program to run upon startup: HKLM\Software\Microsoft\Windows\CurrentVersion\Run (or RunServices). Or, HKCU\Software\Microsoft\Windows\CurrentVersion\Run (or RunServices) Look at your own computer for the format, and name the trojan something important-sounding.
    If you have access to the root directory, you can also edit the autoexec.bat file, and tack the name of the the trojan after win.com, but this method is more visible to lots of users. If you must, you can use this once, then have the trojan edit the line and then add itself to the registry.
  4. Wait until the date. It's probably advisible that you cancel the account you used to plant the trojans with, but that's your problem to deal with.
Obviously, the best way to make this procedure faster would be to program an automatic port scanner that looks for r/w shares on the C: drive, and then does the trojan installation edits automatically.

The beauty of this method is that the owner of the computer never has to click on a program to install the trojan. The tradeoff, however, is in the time it takes to harvest a large number of moron-owned computers. This isn't the only way, but it's probably the easiet and most likely to work. Get a couple other people to help you, and you can easily take down the servers of a large company, costing them (in their estimates) millions.