How exactly did you remove that second OS?
If it indeed is gone, but for some reason at startup you still get the option to select which operating system you want to boot to, then you can change that. I use WinXP Pro, and so if you have XP Home version, it might vary slightly.
Go to Control Panel>System (or shortcut via keyboard is WinKey+Breakkey)
Select the Advanced tab
In the Startup and Recovery section, click on Settings
Now you either edit the boot file if you feel comfortable doing that, or you could just decrease the number of seconds the computer waits for you to select the OS to boot. Do 1 second or so and you will just boot the "default" OS, assuming that is the OS that was left after you removed the other one. This is what my boot file says:
[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