There are a couple of ways to do this. You could use MSCONFIG to disable the startup entry completely, or you can delay it for a set amount of time (in seconds).
Disabling the startup entry using MSCONFIG
You can access this utility by hitting Start and either typing
msconfig and selecting it on the list, or by clicking Run, typing
msconfig and pressing OK. What is msconfig? Msconfig is a system configuration utility provided by Windows. Using Msconfig you can disable Steam from launching at startup by clicking the Startup tab and unchecking the box next to the name Steam. Do not touch anything else, and press the okay button when you are done. It is up to you if you want to reboot or not when prompted. There are other ways of doing this, but are not recommended because that involves messing with the registry, and that's not something you want to screw up.
Delaying the program using an third-party program
Using some of the programs listed here, you can delay the time that Steam starts after your computer boots up (in seconds).
Stopping Steam from launching at startup via Steam
You can also disable it using Steam, if you don't like messing around with stuff that you could screw up. Go in steam, and click the Steam tab. Then, click settings and the
INTERFACE tab. Uncheck the box that says "Run Steam when my computer starts".