Hello vhoi, good morning. 
To beginner Wan: How to delete the other OS? Folow this steps:
1. Right-click on My Computer then Properties.
2. Click the Advanced tab then click the Settings on Startup and Recovey.
3. Click the Edit button then you will see this format:
[boot loader]
timeout=1
default=multi(0)disk(0)rdisk(0)partition(2)\WINDOWS
[operating systems]
multi(0)disk(0)rdisk(0)partition(2)\WINDOWS="Microsoft Windows XP Professional" /noexecute=optin /fastdetect
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /fastdetect
Kung saan ka makaka-access sa Windows huwag mo yong i-erase. Ang erase mo lang yong hindi ka makaka access sa Windows.
Then after that, save it. Then restart your computer then your done.
Take care and God bless. 