Does it recover if you wait ? Is it just on one particular application that this happens ? How much memory have you ?
There could be many reasons for this...
Have you run a disk cleanup and browsing history cleanup, http://windows.microsoft.com/en-US/windows-vista/Delete-files-using-Disk-Cleanup
If you look in "control panel" and select "classic view" at the left hand side, have a look at "problems and solutions" to see what error have occured.
Can you use task manager or Vistas resource moniter to see if any programs or processes are running and hogging the CPU causing slugish behaviour.