What antivirus software do you recommend for the E6400?

NONE.
Just install Windows 7, don't download/run crap, and you'll be perfectly fine. I'm serious.

 

Kaspersky is CPU hungry AV, you need a realy straight hands for configuring it to eat less of system resources. OK, in general, no AV is perfect - I used a lot of AV - every of them has a weaknesses...

 

I'd second that. Norton 2009 has a really light impact on my system and amongst the best detection rates.

 

I agree with this too If you use common sense and don't download things you shouldn't, then there's no reason to end up with malware on your computer. I haven't used anti-virus software for years and I've been trouble free (it's not gloating, many people do this, even some of Microsoft's senior employees).

That said, I'm beta testing Microsoft Security Essentials and it's great. Very simple and unobtrusive, but very little impact on system resources. It's exactly what anti-malware software should be like. You may still be able to download the beta via Microsoft Connect.

 

Using McAfee Virusscan Enterprise for nearly 10 years, runs perfect.