Assuming u have bootable disks u will first need to go to bios (by pressing delete key during bootup) and in the boot device priority place cd-rom/dvd-rom as the first priority bootup device , insert ur bootable disk , press any key when it says press any key to boot from cd , u will be taken to the menu after few mins , select the option for install new copy , select the partitition u want to install the OS on (hope partitions are created on ur drive else dat wil be anoder procedure for u) , and say quick format the drive (preferably using ntfs format as incase u need to copy single file greater then 4.7gb fat32 will not let u ), pc wil restart then let the os bootup dont press any key in the boot screen , select ur timezone wen asked for , enter ur credentials (machine name , password if u want to set , network name if u want to set) , enter valid cd key and let the installation finish .
Incase its ur first time would recommend you to call ur friend and have it done for u .
Install windows xp then windows 7 .