Most probably you downloaded more resource intensive applications with or without you running at background which slow down the whole system.
Normally i would go start->run->type "msconfig"->startup tabs
and remove unnecessary applications especially those from c:/program files/
if you do a alt+ctrl+del and click task manager, you can check on the number of application running on background. Many are not really essential every time the system boot up. Thus you can actually limit the applications running on background.
As for those system applications, if you are not sure of their function or impact visit this website : process library
Usage : Just copy paste the process name from task manager and input into the search field. Base on the return you will be able to judge if you really need that application running on background.
