The easiest, and lazy way is, right click My Computer, Properties, Advanced tab, Startup and Recovery. In the top section for operating system, make sure that the default OS is "Microsoft Windows 2000 Professional" /fastdetect. It should be already.
Then underneath that, for "display list of operating systems for how many seconds", change the number to zero. Click OK. Next time you start the computer, it will boot straight into w2k.
This way, the 98 boot option is still there, but it's just not shown on boot up.
If you want to be neat and remove 98 from the boot menu altogether, you need to edit the C:\boot.ini file.
In My Computer or Explorer, double click your C drive. If you don't see the file boot.ini there, click Tools, Folder Options, View tab.
Then (1) tick Show hidden files and folders (2) untick hide file extensions for all known file types, and (3) Untick Hide protected operating system files. click OK.
You should now see boot.ini.
Double click boot.ini. It should open with Notepad.
Make sure the line that starts with "default=" refers to "Microsoft Windows 2000 Professional" /fastdetect
If so, go to the section [operating systems]
Delete the line that reads C:\="Microsoft Windows 98".
That's the only change you need to make to boot.ini
Save the file and exit. Next time you start, you won't see 98 as an option.