but every time I restart the computer, task manager has been disabled again.
This means that somewhere, somehow, there is still a program being run on system startup that does this. The spyware has not been uninstalled successfully.

In your system registry, check HKey_Current_User/Software/Microsoft/Windows/CurrentVersion/Run for any programs that should not be starting when the system is booted. Also check HKey_Current_Machine/Software/Microsoft/Windows/CurrentVersion/Run for the same thing. If you find anything there, remove it. Make a backup of that key first so if you fubar it, you can restore it to its original state.

It might have also installed itself as a system service. Under the control panel/administrative tools, open up Services, and check if there is something there that shouldn't be. Make sure to remember which services you disable, if any, since if you disable the wrong ones, you will ened to re-enable them, or your system may become verrryy unstable.

And of course, check the startup folder for anything that shouldn't be there too.

If you find a file that any of these searches refers you to, delete it and see if it reappears when you start the computer. If it does, try deleting it and then placing an empty file with the same name in its place with "type nul > filename". Sometimes that works too.

I would also like to restate that HijackThis and this tutorial can be of great assistance in this matter.

Also, some trojans are very effective at hiding their presence. Simply because they do not appear in the task manager does not mean they are not running. Start the system in safe mode before you clean it (even still, you can't be 100% sure) and check there. It's more likely to not be running in safe mode... but not for sure.