chances are youve got alot of junk on it. heres what iu use to clear out pc's
run an anti virus scan (i personally use microsoft security essentials, its free and does the job for me)
run ccleaner. it will clear out alot of junk on your computer and fix the registry.
click windows icon -> control panel -> uninstall a program. uninstall all the programs you dont want or recognise.
click the windows icon. type in msconfig, click startup and then untick everything you dont want.
download adwcleaner and run it after the scan and clear restart the pc.
after you do all this your pc should be as good as new