This is copy paste from another site ,the OS's are switched around but the cure should work for you
Give dual boot the boot
Q: I installed Windows XP on a new hard drive in my PC, but I kept the old drive containing Windows 98 SE as my D: drive. Windows XP apparently detected the 98 SE operating system on the old drive and now offers me the choice of dual-booting at start-up. How do I get rid of this option?
A: You'll need to edit the boot menu, which is controlled by the C:\boot.ini file on your Windows NT, 2000, or XP boot drive. This small text file contains various parameters that control the boot-up delay (Timeout=), the default operating system, and information about which drive and partition contain your two operating systems. Your current boot.ini file probably looks similar to this:
[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 Home"
D:\="Microsoft Windows 98"
To access the boot.ini file, click Start, right-click My Computer, select Properties > Advanced, then click the Settings button under Startup And Recovery. In the Startup And Recovery dialog, click the Edit button under System Startup. This will open the boot.ini file with Windows' Notepad program.
To open the file without the long string of dialogs (using Notepad or another text editor), you must first remove the file's read-only attribute. To do so, click Start > Run and type cmd. Click OK, and you'll see a command-line dialog. At the command prompt, type CD\, then press Enter. Now type attrib -r -a -s -h boot.ini (note the spaces between command letters and the hyphen for the next letter) and press Enter. This will remove file attributes that would prevent you from editing the file. Next, type notepad boot.ini and press Enter to open and edit the file.
Now, to remove the Windows 98 boot option, simply delete this line: D:\="Microsoft Windows 98." Without the line, Windows XP becomes the default start-up OS. If you prefer to leave the dual-boot feature as is, but you want Windows to start the default operating system sooner, simply replace the number after Timeout= to something less than 30 seconds. (Windows will present you with a dual-boot option, but it will launch XP within the time you specify.) When you have completed your edit, save the file and close Notepad.