Most likely caused by the virus.

This link has a quick collection of ideas to restore that function, though once again I would say run in Safe mode and try getting the tools (Malwarebytres, AVG, Ad-aware) to do the work for you.
http://www.pchell.com/support/registryeditordisabled.shtml

In all cases I would try downloading fixes on the unaffected PC and transfer them to the infected machine by flash drive.

Is there a reason rebuilding the infected PC isn't next on your to-do? You've put in some time on this without a victory. While fighting this type of problem is often educational, I wouldn't trust a recovered system that was this badly infected for very long. The only reason to keep fighting for a 'cure' is if there's something about the current system's config you want to retain.

 

Perhaps I could take it to a professional? I can't really shell out for a new pc at the moment, this laptop is quite slow and old, and it is used by more than one person.

I was going to take it to a professional first but I thought I may be able to remove the problem myself.

 

You shouldn't need to buy a new one- just reinstall the OS on this one, wiping out the drive (and its infection) in the process. You'll need to load the PC-specific drivers and any additional programs afterwards, but it would probably operate better than it has been since even before the infection.

Assuming you have the disks, this wouldn't cost you anything but time.

If you want to go this route, first copy the C:\Documents and Settings folders over to a flash drive, so that you might be able to restore the other users' data and settings when you're done.

 

In any case make sure to back up your information. And going to a professional might or might not fix the problem because might just do the same things you just did and if it fails again then they might recommend a reinstall of OS.

 

No registry editing?

Sounds like you caught a nasty one. I somehow managed to get Win 32 Vitro a while back and it was a small nightmare. It would actually prevent my browsers from going to ANY kind of anti virus website. It apparently knew what to look for and wouldn't let me load the sites.

Still, it did still let me edit my registry.

If you end up wiping the drive and reinstalling the OS, make sure to check the boot sector. Certain viruses can lurk there, even after a wipe.

 

If it does, run DBaN, as it formats completely and clears boot sectors.