Are you using a wireless mouse ?
Ctrl - Alt -Del is used in Windows 2000 or XP to pop up Task Manager, so of course if there's something wrong with Windows that doesn't allow the former to work, the latter will not work.
If your computer has a Reset button, pressing that will usually not work either in that situation,
Usually, due to default bios settings, if you hold the Power button in (or down) for at least 4 seconds, the mboard will shut off.
Try starting up Windows in Safre mode or Enable VGA mode, if you haven't already.
Remove any bootable CDs or DVDS in drives.
Press F8 repeatedly while booting the computer, do not hold down the key, and when the Windows loading choices menu appears, choose Safe mode or Enable VGA mode or one of the other choices
Some brand name computers or mboards on generic systems have another use for F8, e.g. it may pop up a boot device selection menu for the mboard . If you see something else when yoiu press F8, press whatever key exits the sxcreen, then immediately type F8 repeatedly again