Press the Windows key + Pause|Break {this will bring up the system properties dialog}. Go to the Advanced tab. In the section titled Startup and Recovery, click Settings. In the dialog box that appears, look in the section titled System Startup. You will see a button called Edit, click this. It will open up the boot.ini file in notepad. Under the [operating system] portion, you will probably have two lines. Delete the second line if that is the operating system you no longer use.
You want it to look something like this:
Code:
[boot loader]
timeout=30
default=multi(0)disk(0)rdisk(0)partition(1)\WINDOWS
[operating systems]
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /fastdetect /NoExecute=OptIn
Jam it back in, in the dark.