Ctrl+Alt+Del as well as Ctrl+Shift+Esc should bring up the task manager in xp. If not, type taskmgr.exe in the Start->Run and see if you get it... If you do, it may be a lost shortcut association or something (dont know how to set that right, but you can create a shortcut to taskmgr and give it a shortcut key)...
If not, go to System32 folder in Windows and copy taskmgr.exe as taskmgr.com and try to run it... If it works, you may be infected by a virus... Go to Safe mode, run virus scan and spyware scan...
Also, run SFC to test if any system files have been changed/corrupted...
Arun