Hi:
This doesn't answer your specific question or address your specific issue, but...
...Windows Defender (WD) under Win7 is just a weak anti-malware/anti-spyware application.
It is NOT an anti-virus (AV) application on that platform.
Even with WD running, you would still need a real-time AV, either free or paid.
And there are better choices for real-time anti-malware protection to help prevent infection alongside your AV.
So,by itself, I would not worry about WD's being broken.
Having said that, you said: "No other antivirus programs installed".
If, in fact, you've been running with only WD and without a bonafide, real-time AV, then it's possible (likely?) that you may be infected and the malware could be preventing you from turning on WD.
You may need to get some help running some scans to see what's lurking on the system.
Aside from WD being disabled, what other signs/symptoms/abnormal behavior are you experiencing to suggest possible infection?
MM
