|decrease page file.|
decrease storage space allocated to the recycling bin.
in c:\windows, show hidden files and delete all the folders with names like "$NtUninstallKB958687$." Doing so will prevent you from ever uninstalling any Windows Updates you have installed, but this is rarely necessary and typically, these can be deleted without much worry.
you can also do an advanced search on your whole C:\ drive, but only search for files changed or added in the last 7 days, 2 days, whatever, and then sort the results by size. This might give you an idea why your free space is decreasing, what file types are growing or being added, and where they reside.
Once all that is said and done and you can relax again, start thinking about the next step: getting a larger hard drive. you will need to either a) do a fresh install and migrate your data files, or b) use a 3rd party software application like Norton Ghost or Acronis to copy your existing OS installation and push it onto a larger disk.