Yes, for the best coverage you'll need to install AV software on both partitions. You'll either have to purchase a version that is compatible with XP, or download a free one. Maybe someone else can suggest some good AV software that is free.

You can try Avast or AVG.  Both are free and are supposed to be much lighter on resources than Norton.  I would even replace the Norton on the Vista partition with a less resource-hungry anti-virus that is aso free.

Ok! Thanks alot!

You are welcome.