Could be a program or a service, Task Manager will show what's currently running and if it's using cpu power, those would be the most likely ones.
Also in Vista try turning off disk indexing, it speeds it up no end (Start>run>services.msc,run it then scroll down to Windows Indexing Service, right click, properties, stop and then select "disabled")
Free programs like Cleaner also have an easy to use way of disabling programs/services that run at start-up and you don't need/want.