Vista missing Windows Defender?

I've Googled this a few times with no results.

I'm working on an older Acer laptop that has Vista Basic on it. It had a lot of malware, which I've removed. However, Windows Defender doesn't work. It had some sort of 0x000etc. error when I tried to launch Windows Defender from the Control Panel (Security section).

I tried a few fixes I found on Google, and NOW... not even that error. If I click on "Windows Defender", I get the error "Application not found" with "Explorer.EXE" in the error window title. If I click the sublink below Windows Defender ("Scan for spyware and other potentially unwanted software"), I get "Windows cannot find C:\Program. Make sure you typed the name correctly, and then try again."

I downloaded an offline installation file for Windows Defender, but running that simply lets me know that it can't install, because Vista already has Windows Defender.

How do I fix this? I'm really at a loss. I've tried the registry fix scattered over the internet, does nothing. All related services are started and automatic (I believe RPC and DCOM are the services, going from memory). Please help!

 

Bump

 

Is the machine also running MS Security Essentials? If so it disables Defender.

 

No, it's not running MS Security Essentials or anything else that could conflict with Defender.

Either way, while a solution to this would be nice to know for future reference, I can't wait any longer and am just going to reinstall the OS.