The best way to run a (Windows) computer securely is to run it on a standard account. That way, no program can do anything damaging without asking for an administrator password first (as long as you have UAC on). This is how Unix systems run, and it's the reason there are no effective viruses for them.
So, to summarise:
1. Make an administrator account with a password
2. Convert your account to a standard account
3. Remove all antivirus forever! (Actually, don't. Run a scan every now and then, just in case)
Jordi said:
I use AVG and I can't remember the last time I had a virus. It must have been years. That doesn't necessarily say anything about AVG though, because maybe I'm just careful or super lucky and haven't encountered any viruses to begin with, or I have but other AV software would also have protected me.
Anyway, I have no reason to switch. With AVG I've always been safe, it is free, it is unobtrusive and if Task Manager is to be believed it uses virtually no resources.
But task manager doesn't tell the whole story when it comes to antivirus. The way they work is to scan every single byte of data that passes through your computer for anything harmful, which means that every network packet and every disk access attempt has to be stopped, scanned, and then allowed on its way.
There are some (slightly out of date) benchmarks for this with different software packages [a href="http://thepcspy.com/read/what_really_slows_windows_down/#results-and-conclusions"]here[/a].