The source of 90% of all computer problems stems from somewhere between the keyboard and the chair. LOL!
I'm not in any way advocating no protection but if you were careful enough, technically you could get away without it.
As for choice, everyone has their own take on it so it's pretty much down to whatever floats your boat.
One thing I will say though,
Some of the bigger players in the marketplace are actually the worst....
In fact some of them only protect you because they grind your machine to a hault leaving it incapable of functioning properly. A bit like, "Less function = less chance of infection".
I'm referring to some of the big names like N****n & Mc**** which are both owned by S******c & who's only claim to fame is that either one will grind your system to a snails pace & are nothing short of a joke IMO.
Some of the free options out there are far superior (Again just my opinion).
Personally I use AVG (Paid version because it has it's own Firewall etc).
Plus it gives a 2 year X 3 machine licence at a very reasonable rate.
MalwareBytes (MBAM) is always close at hand too ;-)
Mischel Internet Security do a reasonably good product called Trojan Hunter & it runs nicely alongside your main Antivirus or Internet Security suite.
As for SPAM in the email sense, the best I have encountered is a package called Mailwasher Pro, exceptionally good for anyone using POP3 to access their mail. It lets you see all the incoming stuff right on the server before you ever accept anything, basically you get to stop it at source & not after it has arrived on your system.
There is no definitive rule when it comes to PC security so I'll only suggest the following,
Find something that delivers the following,
1. Free or at least reasonably priced.
2. Known for ease of use (No 3rd level qualification required to use it).
3. Easy on system resources (Doesn't grind your system to a hault, in the name of protection)
4. Updates regularly.
Most importantly, NEVER EVER
go by the vendors description, if you were to believe any of their crap you'd imagine that they could cure the recession. Always Google for users reviews & ask other users what their experiences have been with different products.
I have some machines here that have been running on AVG FREE & Windows own Firewall for years & have never been infected by anything serious.
I mean, "Hello! You are visitor number 1,000 000 & you just won a million dollars" Click on me to claim.........
Some people actually do click on the button
Box clever & you'll be alright. :)