You need to edit a file in the root of C:\ called "boot.ini". It is a hidden, system file so you will need to enable them in "Folder Options".
Just use notepad and delete the second line of Windows 2000 Professional.
Should look something like this:
[boot loader]
timeout=10
default=multi(0)disk(0)rdisk(0)partition(1)\WINNT
[operating systems]
multi(0)disk(0)rdisk(0)partition(1)\WINNT="Windows 2000 Pro" /fastdetect
As long as their is only one choice, then the clock will not show up, but you can set the "timeout" to 0 if you don't want it to ever show up again.
EDIT- Of course, you could take the easy way out and do it the way GZ3 says...