Long answer: AVG manages your viruses - Defender manages your spyware. Vista is naturally slow - not because of programs; but because of the way it manages memory. I'm one of the many who say downgrade to XP
Yes, you DO need a good antivirus and spyware software to be present on your system. You also should have a good quality firewall as windows firewall is just not good enough. And as for Vista or XP... XP always!
I'm going to be the dissenting voice here I'm running an HP nc6400 laptop with 2GB memory and Vista Ultimate (the laptop is designated "Designed for Windows XP, Windows Vista Compatible" and I put a new install of Vista on it).
I've got no problem with performance, it performs pretty much the same as my original XP install. I also use a non-MS anti-virus program (Sophos SBE) alongside Windows Defender and I think both are appropriate, though AV vendors do make varying efforts to remove spyware I haven't seen a measureable matrix for this yet (unlike the VB100 awards).
I also think the Windows Firewall is sufficient for a system as long as it's updated regularly, and you stay off illegal download and hacking sites. I've not had a single security problem in 3 years using XP/Vista, one AV product and no extra firewalls. This assumes you're behind some kind of NAT firewall/router which 99.99% of users are.
I have AVG Free edition on my main machine, and I've left Defender installed (the pc runs Vista Home Premium)
As to firewall, well that depends on your connection to the internet. Do you use a router? If so you'll often find that with both NAT and SPI you don't need to bother with yet another software firewall. The XP or Vista firewall is perfectly adequate.
However, my customers biggest problems originate when they are surfing for pron. Give it up folks, you'll wreck yourself and your computer! Sorry off topic
I don't notice any performance hits, and I've been supporting Vista since November 2006...
I have always used Windows Firewall, AVG and Spybot and have never (touch wood) had a problem with a virus in all the years I have been using it. I also use a router so don't know really if I have too much security! My PC runs Vista Home Premium.