did you check msconfig? (assuming winxp/2000) If not go into run and type msconfig, those are the programs windows loads, other than those the system requires. Some other sund/video things are in there aswell so choose wisely..
Win2000 didn't come with MSconfig, none of the "NT" systems did. Hit Alt+Ctrl+Del and just see which program is using the CPU (only works if running XP or 2000)