You can always do things like that in safe mode. =P
To turn it back on, you could try rebooting in safe mode. There should be a second user, called "Administrator" You should be able to edit the settings of your normal user under "User Accounts" in control panel.
Norton and Mcafee both suck ass. Microsoft has a "Malicious Software Removal" tool in XP, upgraded through Windows updates, but it's a piece of shit that does little.
Alot of viruses will be running and actively prevent you from getting rid of it, normally the first step is to open task manager and find the process, end it, then continue to find the virus. As well as removing it from the registry. However, as you said it had turned off task manager, you should go into Safe Mode. =P It won't run at startup in safe mode, and you can log into the administrator account.
I know you said you already got rid of it, but with viruses come adware and spyware. I would make sure you have gotten rid of everything. Find a good anti-spyware scanner and rid yourself of them, and a good anti-virus. I use Spyware Doctor and Avast! personally, as well as Spyware Begone for extra protection. Another good step once things are fixed, go into safe mode with networking, visit Trend Micros website, and use their housecall. It's free and it often gets a couple missed things.
