Go to Start > All Programs > Windows Update
You will be presented with high priority updates:
By default they will all be checked but you can have a look through them and uncheck them. You can then select from the optional updates.
When ready select Install Updates.
The updates will begin to download and then install.
Continue this until you get the following screen i.e. no high priority and no optional updates.
Windows XP – End of Life – Extended Updates
Note as Windows XP has reached end of life you may get a warning about Windows XP End of Life.
Although Windows XP has reached End of Life and is "no longer being updated" some corporations have paid for continued updates,
Windows Embedded POSReady 2009 which will continue to receive updated until April 9, 2019 and a hack to change Windows XP into thinking it is Windows Embedded POSReady 2009 will allow for some more security updates. The Unofficial XP Service Pack 4 Update includes this registry edit.
To do this open notepad by pressing [Windows] and [r] and typing notepad
In the notepad write
Windows Registry Editor Version 5.00
Go to File > Save As…
Ensure to select all files and save it as XPUD.reg
In Windows Explorer double click the XPUD.reg
Select Yes to add the information to the registry
Open Windows Update to obtain the new updates:
Registry Edit taken from: