first off....laptops are a lot slower when running on battery...and also some remain slow when plugged in to a power source because of power settings....unless you are using huge harddrives then how much free space you have is really moot.

slowness is usually caused by low memory issues. Doesn't matter as to how much RAM is installed...what matteres is how much PHYSICAL RAM is AVAILABLE
start > programs > accessories > system tools > system information and scroll down to read how much memory you have installed and how much of it is free to be used. If you have less than 25% or so of installed memory left available then you need more ram... Remember that Vista requires a lot more memory just to make it look pretty to you!

The next biggest thing is malware/spyware/virus issues....make sure your system is clean..

Next look at what programs are running in the background or during start-up(these will continue to run in the background) All installed programs are built with the pretense that they should constanly monitor itself and what you are doing. This is just plain false....with a little googl help...find the proccess for each ADDED program(not system proccesses) and set it to manual and not AUTO. I use WinPatrol (free) to help monitor these program processes.

BillP Studio: WinPatrol Download

Camera and picture programs need only to run when needed same with printers and scanners...
video/audio the same thing....turn off auto updating to every program and teach yourself to search for updates on a regular basis (monthly is suitable for all programs ) The only thing you need auto updating is your ant-virus /spyware/ malware programs.

If you are using torrents to download...make sure that you are not seeding as required by most torrent sites.