Windows Security Alerts

My friend had AVG Security but let it lapse. When she booted up her PC and tried to access the internet she got a message saying that various malware had been installed and she had to buy a security package to remove the malware and protect her computer. I think the package was called "Internet Security Pro 2013", which I now understand is itself a piece of malware. At the time, I thought this looked like a scam so I advised my friend not to accept buying this package. Instead, after a lot of messing about and trial and error I managed to get AVG reinstalled and this seems to have removed the malware. However, when my friend boots up her computer now she gets a message from the Windows Security Centre which says "... make sure the four security essentials below are marked On or OK." The four security essentials listed are:-
Windows Update
Windows Firewall
Windows Defender and
Windows Security Centre
However, when you try to turn any of these on you are unable to. For example, the screen says that the Security Centre is turned off and there is a button which says "Turn on now" but when you click it you just get a message saying "The Security Centre Service can't be started." The computer seems to be running fine but I'm wondering if these messages mean it is still vulnerable in some way.

My friend's computer runs on Windows Vista Home Premium
Intel Core CPU 6400 @2.13 GHz
2.00 GB RAM
NVIDIA GeForce 7500 LE