was your pc like this ever?
has it started using so much of ram recently?
if yes
what programs you have installed recently?
task manager can show you which programs / services are taking so much ram
start task manager, sort the grid by memory usage, my vista says iexplore.exe is using the maximum memory
is the cpu usage also high?
in case you feel that some application is doing this
analyse which applications are running by default?
use autoruns by microsoft to find out which applications run by default
how much memory it consumes in safe mode?
does your vista shuts down properly?
hope answer would give you a clue