kaspersky or mcafee.
Both are fine. And in the last year, tons of trojan virus are found from
emule files. Those trojans can block mcafee and kaspersky. If this happened,
use system restore or system repair from installation disk. They can fix
most problems.
In many cases, if you wanna improve the performance of the system, you can
try use high speed mode of realtime scan and turn off many useless functions
like email protection. But a firewall is always necessary.