Recently I deleted around 60GB of crap from my computer. Bloat ware, old games, old files, cleaned out my program files, used the uninstaller in the control panel, etcetera, but my hard drive space is decreasing daily. No I don't use Norton and yes I deleted the files in the Recycle Bin.
It is pretty normal for you hard drive space to slowly decrease over time.
As your system runs it will generate temp files as needed, create new system restore points, automatically update Windows and other software, ect.
Try using CCleaner to clear out the junk once in a while and do not worry to much about it.
If your lack of free space is of major concern you could always upgrade your laptops hard drive.
Current laptop drives go up to 1TB in size and are probably a bit faster than your old 320GB as well.
You can change the amount of disk space windows uses for restore points and for the trash. In "disk cleanup", I believe in the advanced tab, there is the option to delete all but the last restore point. As long as you know/think you won't need those old restore points, it's safe to delete them.
I also recommend windirstat. It shows a graphical representation of your files and folders so you can see what's eating up the most space.