As an afterthought you may want to check for spyware while we are at it.
The free version of this tool will do as good a job as any:
http://www.superantispyware.com/superantispywarefreevspro.html To check whether or not you have a service pack right-click "My Computer" and select "Properties" and it should say so there on the General tab.
Or...go to Start/Run and type in winver then click the Enter key. It will spell out exactly the version of XP you have.
I would imagine you would know whether or not you installed any service packs, surely? They are huge. If you don't have any, then your system is unsupported by both McAfee and Microsoft and you should be installing SP3 which includes all updates since XP started until about April of this year.
Help with that is available here:
http://community.mcafee.com/showthread.php?t=220807