First, open a command prompt.  Click start, then run and enter cmd, then click ok.
Rnter the following command to see the boot info "bootcfg /query"
(this will display your boot settings, what you will need is to take note of which boot entry ID you wish to delete.)
Then type the following "bootcfg /delete /id 2
(In this case we have chosen to delete the second OS entry which we noted down earlier from the bootcfg query command.)
Then once you restart you won't get the option to pick what OS you want to load.
The above is good if you uninstalled an OS but XP still queries you which OS to load.