|You need to provide a lot more description about what you can and cannot do in Windows, and any other symptoms you have.|
Some malware can disable you being able to use Task Manager, but you haven't provided enough info for us to conclude that that's probably your case.
Your user (account) probably needs to have administrator rights in order to be able to use Task Manager.
All users of XP have administrator rights by default except the optional Guest user which always has limited rights, but someone with administrator rights may have assigned limited rights to your user (account).
If you are having problems with your keyboard, or if your keyboard type is set wrong, holding down Ctrl and Alt then pressing Del , letting go of the keys, may not pop up Task Manager.
You can also RIGHT click on a blank part of the Taskbar at the bottom of your main desktop screen, and click on Task Manager .
If you get an error message that indicates you're not allowed to use Task Manager when you try to use it, either your user (account) has limited rights, or you have malware in Windows.