|Which OS is on it now...Windows 7? Why would you want to replace it with XP?|
The 1st thing you need to do is go into the BIOS & make sure the CD drive is listed before the HDD in the boot order....if it's not, change it & save the settings. Also, XP doesn't natively support SATA HDDs so you'll have to do one of 3 things to get it to work:
1. go into the BIOS & change the controller setting to IDE compatibility mode. After XP is installed, install the SATA drivers, then change the controller setting back to SATA mode again.
2. load the SATA drivers at the very beginning of the XP installation by pressing F6, then installing the drivers from a floppy or USB device (if supported)
3. create an all new XP disk with the SATA drivers slipstreamed into it.