When Dual Booting XP and Vista?

Do I need to install antispyware software on both operating systems or will a single installation serve both? I'm currently running adaware and spybot from XP.

When Dual Booting XP and Vista?best antivirus software

One should do it, just make sure it scans all your drives. Vista has a new disk encryption however that may prevent it so to be safe run 2.

When Dual Booting XP and Vista?firewall

I'm not sure if an antivirus could monitor both, but for scanning you should be able to select both partitions to be checked.
yes, you need to install programs on both os's. Each os can only access programs installed under it. It's like the U.S government can only tax U.S citizens in the U.S, and not Canadians, os work that with programs.
yes, you will need to install antispyware/antivirus software on both operating systems separately
You would need to install apyware/adware protection on both systems due to the separate partitions.
U need them on both!
You need one on both installations. As spyware can infect the registry and the antispyware can't scan the registry from one operating system to the other operating system. Good Luck HTH.