If you do not have many startup items, you won't notice much difference.
Selecting a lower number of processors to use at boot up than you actually have could increase the boot up time causing a longer time to boot.
Here is an example of my boot up time before (1 core) and after (4 cores)
Before (1 core):
After (4 cores):
Ok, so here's how you do it:
Open the Start Menu >> In the search box type msconfig >> Enter >> Click the Boot tab >> Advanced options.
To Change the Number of Processors used at Boot, check the Number of processors box, then click on the drop down arrow and select the number of available processors you want to have used at boot up.
note: To test to see if this can speed up your boot time or not, then select the highest number listed for all of the available processors to be used at boot up and check the Detect Hal box.
For me I will select 4 cores on my Intel Core 2 Extreme X9650
Check the Make all boot settings permanent box in the bottom left >> OK.
A window will pop-up, click the Ok button. You can change setting after that if you want to >> Check Don't show this message again >> Click Restart
If you want to test your boot up time, you can read this tutorial: Test Your Computer's Reboot Time
There you have it, super fast start up. Enjoy!
To Restore the Default Number of Processors at Boot >> Uncheck the Number of processors box >> Click OK