Take a look at what's running at startup. Go to Start>Run. Type msconfig and click 'OK'. Select the Startup tab. If there's anything you see there which you don't need, click to remove the tick alongside it. Click 'Apply' and 'OK'. Restart your PC. You'll see a message indicating that you're using 'selective startup'. Click to accept 'Do not show this message again'. (You can reverse any changes by simply going through the same process and putting the ticks back).
It might also be worth checking the settings in your anti-virus software. It's common for such software to run a full scan at startup (which can slow everything down until the scan is complete). You might want to change the settings so that it either carries out a more basic scan or only scans when you ask it to.
Chris