Windows Defender - Anti virus? or not ?

[#1]

Running Windows Vista Home Premium

Just checked my Windows Security Centre.... and it states.....

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_________________________________________________________________________ Malware Protection                                                                                    Check Settings __________________________________________________________________________ Virus Protection                                                                                           Not Found Windows did not find antivirus software on this computer _________________________________________________________________________ Spyware and other malware protection Windows Defender is actively protecting your computer __________________________________________________________________________

So it would seem I DO need to have a third party AV installed as well as Windows Defender...
Which I removed to try and improve my Youtube experience...  

EDIT EDIT EDIT.....
The choice is AVAST at 87MB  or AVIRA at 124MB...... comments on your preferences Please ?

[#2]

I'd remove Defender and install Microsoft Security Essentials

[#3]

I think MSE on pre Windows 8 is AV while Defender on Win 8 and 8.1 is AV

[#4]

OK... Thanks for that DomS...... ( and PW for your advice )....  That is where I got a bit mixed up then...... down loaded and installed MSE and it has just run a quick scan... all is good..!.. so lucky me ! ! ! ..

[#5]

FWIW: Installing MSE on Windows Vista or Windows 7 will automatically disable the original built in Windows Defender as its no longer needed.

[#6]

And trying to install it on Windows 8 results in a message saying it isn't needed as Defender is the new MSE or something like that.
I use MSE on my Win7 and Vista machines and Defender on my Win8 (cough spit) laptop.
Yup - just tried to install it again - got the message :

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You don't need to install Microsoft Security Essentials Your version of Windows includes an updated version of Windows Defender that provides the same level of protection as Microsoft Security Essentials, along with other significant improvements