Before playing with the Registry try these general tips to see if it solves your problem.
See if System Restore will get you back to a restore point before your problem with Task Manager.
Run the System File Checker program from the Run Box by typing.....Sfc /Scannow in it and have your XP CD handy.
If they don't work you could try repairing windows itself by running it over itself. You will lose all your windows updates but your files will be untouched.
How to Perform an In-Place Upgrade (Reinstallation) of Windows XP (Q315341)
If you are using Home you wont have GpEdit.msc (Group Policy Editor) on your machine. If you have Pro it resides in the System32 folder.
< When I want to do a regedit, for example, I get the msg "Task Manager has been disabled by the Administrator">
Do you have access to RegEdit??????
The following is highly speculative and may or may not help you.
Use the download to set the key up and then find the key and change the value to allow access. The download is to disable Task Manager.
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\System]
This is the location of the registry setting after you run that download.
PLEASE BACK UP REGISTRY BEFORE ALTERING ANYTHING OR APPLYING DOWNLOAD.
322756 - HOW TO: Back Up, Edit, and Restore the Registry in Windows XP