I installed MSSE to shut up Windows 7, not because I needed an antivirus. They cause more problems than they solve unless you like downloading porn off warez sites. Other than not going to the darker side of the internet, just don't run Flash and Java when you aren't using them.
Unfortunatly there are so nasty things as driveby downloads and browser exploits... yes even without java or flash. The risk is certainly lower but being on the net without any AV is close to being ignorant.
The days where Viruses formatted your disk for lolz are mostyl gone. Today they want your data.... or your computer power.
Some people who state that they need no AV at all received a nice letter from the ISP that kindly informed them that their account has been used to send millions of spam e-mails. The same people wonder why the can find their credit card information on the internet..... or their latest private moment on youtube.
Like any defense, no AV offers total protection.... it however helps you a lot in dealing with the more sneaky variants.... which are the majority today.
There are plenty of good free AntiViruses out there.... who dont consume a lot of system ressources.... and offer good protection.
Although of all free choices going to MS seems like a bad idea.
Microsoft + Security = FAIL
I dont know why they cant do a reasonable product concerning security.... and I really dont understand how some 50 people companies can outclass a gargantulan monster as Microsot who could easily sink 2 billion into development of a good Anti Virus.
In any case... before you have nothing... use the MS product.... its better than nothing.....
Just dont expect it to stop anything malicious that has been written with a minimum of skill.
On an related sidenote..... having a firewall is not a bad idea too. Not only it does add another layer of protection but it can also be used to shut out the annyoing PAID products that phone home... and then sell your data to the highest bidder.
If you are interested... I recommend Comodo Firewall. It is free and it is the best firewall I ever had. So far nothing managed to get by it.