Here is an interesting idea: The author Oli has installed a number of software packages on a clean VMWare (one after the other) and measured the boot time before any installation and after each new installed program. He wanted to see which programs slow down a PC the most.
The result: The worst contenders were Norton Internet Security (shudder… comes to no surprise for me) and 1000 truetype fonts (!). Various instant messengers and other security software also slowed down the most.
In closing, just be careful over what you install. If you need a font for something, you only need to open it in the Windows font browser and it should become available in your application until you close the font browser. Installing and forgetting about fonts is a very easy way to slowly degenerate your boot times. If you can avoid them, don’t install an antivirus program (especially not Norton).