It sounds like a worm that was doing the rounds a while ago.
If it is, in addition to the task manager not working, the firewall won't enable itself, anti-virus programs won't start and system restore won't work. Also, it prevents you from starting up in safe mode.
The only way I found to get rid of it was a complete reinstall of the operating system - though I didn't look too hard as I was thinking of doing a reinstall anyway, before that happened.
I don't know what else the worm did, but as it switched off the firewall (not just Windows, but my Sygate too), I can only assume it had installed a remote bot and was planning to use my system to run denial of service attacks along with thousands of others.
After-thought: If it's XP Pro (not sure about Home or any of the Vista flavours), click on Run and type in gpedit.msc - there's a lot of things in there you can set/reset.
Last edited by kenhughes; 19th May 2008 at 22:39.