Taken from Speed Up Windows 7 – Ultimate Guide To Make Windows 7 Blazing Fast!!!

  1. Press [Win] + R to bring up the run dialog, or put your curson in the search box of the the start menu.
  2. Now type msconfig in there and press Enter Key to open up the System Configuration Window.
  3. Click on the Boot tab.
  4. On the right hand side, you will see a box called Time out. This is the time (in sec) the system waits for the user to select the operating system to boot.
  5. If you use only a single operating system (don’t dual boot) you can safely set the value to 3 (the minimum).  But if you have more than one operating system installed then set it to 3 or more (which ever is convenient).
  6. Click ” Apply “and then on ” OK “.

The change will take affect the next time you boot the system.
On a side note, there have been a lot of posts saying that if you use the option to “hard code” the number of processors/cores you have your boot time will decrease.  Microsoft has removed the knowledge base article that made the clam and numerous users have proven the claim to be false.  The option is there for debugging purposes, allowing you to decrease the number of processors/cores that windows will use.  Not something you want.  You can read more about it at Ed Bott’s blog, http://www.edbott.com/weblog/?p=2476.