Sudden freezes, and bits missing

Recently, comp randomly 'freezes' for a minute or so at a time, then returns
to normal. Ctrl/Alt/Del reveals procesor usage approaching 100% at these
times. Can happen when running any application.

Second problem. When accessing control panel from 'Start', it crashes, the
desktop clears and then re-populates itself with icons. This does not happen
when accessed from double click on 'My Computer'

Third problem (maybe related). Dropdown menus no longer appear, eg when
right clicking on IE Explorer Icon to get to properties. The outline of the
drop down box appears, and if the mouse cursor is hovered over the
appropriate area, the menu options appear, and when clicked they still work.
.