There are 2 ways you can do this...
First, make sure you know the directory the XP installation you wish to keep is in...
Go to Settings>ContolPanel>System. Click Advanced Tab. Under Startup and Recovery, click Settings. You should now see a drop-down menu listing the operating systems available on your machine. Choose the one you want to boot into (see statement above) and change the time to display list of operating systems to 0. Restart your computer and your computer will boot (without the menu) into XP.
The other option is to edit the boot.ini file in the root directory. It is a hidden file, so change the attributes. Remove the line that refers to your old XP installation, re-save the file and reboot.
I like the first method myself because it is the old point n' click method. Let me know how you do...
PhatPhunky