Windows stores files you delete in the Recycle Bin, allowing you to recover them in case of accidental deletion. Regularly checking and emptying the Recycle Bin will allow you to save even more space. You can also skip the "delete to Recycle Bin" part of the equation by Shift+Clicking on the "Delete" option when deleting a file in Windows Explorer. This permanently deletes selected files and folders, rather than dumping them into the Recycle Bin. However, be sure that you want those files and folders dead and gone, because once you do this, there's no turning back.
Another space-saving maintenance step is to regularly empty your Temporary Internet Files and other cache-type folders that programs (like your browser) use to store temporary information and browsing history. While temporary files and caches can improve performance and surfing speed, they can also use up hundreds of MB or even a GB or two if not regularly emptied. You can usually automate this in your browser's settings, or manually dump these, along with the Recycle Bin, using a utility like CCleaner.