Cannot launch any program as administrator

Hello.

My computer has a problem. I use an account with restricted rights on Windows Vista, and usually don't use the administrator account.

However, these past few days, I have found myself unable to perform actions that need to be performed as administrator, such as uninstalling a program, lauching a program as Administrator with the right click, modifying the registry, enabling User Account Control and others... Whenever I try to do any of the following, I am not prompted to input the Admin password anymore, or if I'm trying to run a program as Admin, it runs, but not as administrator.

I made another topic on the Hardware forum, but was told to come here because my latest logs don't show any malware. In short, I downloaded something that was supposed to be a keygen. *slaps head* When I double clicked on it, nothing happened, well, on surface at least. Which is why I got suspicious and thought this might be a virus or something masquerading as a keygen.

So I ran anti spyware applications, and some of them detected files they identified as trojan, adware etc. Also, I detected processes with strange descriptions in the task managers. And several of the files detected were actually linked to those processes.

Then, I rebooted the computer at least once, found this forum, followed the procedures written in "Vista & Windows 7 Malware Removal/Cleaning Procedure", and disabled UAC in order to run MGtools.

As I wrote in the other topics, MGtools never was able to create the MGlogs file. Also, when I tried to re-enable UAC after running MGtools, I received this error: "Cannot import C:\MGtools\EnableUAC.reg: Not all data was successfully written to the registry. Some keys are open by the system or other processes."
I also tried re-enabling it through the control panel and security center.

When I try to re-enable through "User Accounts", I don't get a prompt for the Admin password, even though I should, since it has that Windows icon next to it, and am allowed to reach the menu, but even though I check to use UAC and click OK, my changes are not saved.

If I try to re-enable it through the Windows Security Center, I get this error: "You don't have the correct permissions to perform this action. For more information contact your systems administrator."

I hope that someone can help me. Thank your for reading!