If this machine has updated on the past, and if you are current on Windows
Update (i.e. you can do an express scan, and see nothing offered) and you
are on Windows XP (as you've stated!)--then this is how I'd proceed:
Go to a command prompt, in the Windows Defender install folder--typically,
this is \program files\windows defender.
type these two commands, hitting enter at the end of each, and waiting for
responses before continuing. The second command may take some
time--minutes--to complete, and will require Internet access:
mpcmdrun -RemoveDefinitions -All
mpcmdrun -SignatureUpdate