Just install it in your XP just like you install any other program in XP. That is: boot into XP and put the McAfee CD in the drive. I don't know if it's autorun or if you have to run a program like setup.exe yourself, but you'll be able to find out if you do.
You don't tell anything about the license. It could very well be you have to buy a new license, if the old one covers only one install.
Of course, there's no need to use the same antivirus program (in your case McAfee) on both OS'es. You can choose any other you like, either payed (Norton, Norman, Panda, Kaspersky, Trend etc) or free (AVG, Avast, Avira). Totally your choice.
Kees
Hello;
Due to the known compatibility issues of Windows vista with many programes, i decided to have a second Operating System - Windows Xp in one of other drives. Now i have got a registered version of MCafee Antivirus in the Vista Operating system. Vista is slow and i have promptly moves back to Xp as my default Oprating System. But i am unable to extend the Antivirus to this Operating System (Xp). Do help me in this regard.

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