Low disk space: try the disk cleanup wizard. See http://www.aarp.org/computers-howto/Articles/a2002-07-15-tempfiles.html. http://www.toejumper.net/maintain4/del4.htm gives more tips and hints. Also note that not only programs take space, most space is taken up by data (like mp3-files, jpg-files etc). And only delete programs by using their uninstall feature. And compress your Outlook Express folders. Windows Explorer is not a very good tool to monitor file space used, but if you set the view to detail it's usable. My Computer (properties of the c:-drive) shows you the overall picture in a nice pie chart.
It's up to you to decide if your hard disk is big enough for you if you cleaned everything you don't want to keep. A computer 6 years old typically has 2 Gb, a new computer typically 80-200 Gb, and you'll find anything in between. Pick what suits you.
If you've got any doubt if deleting a file is a good idea, the least you can do is delete it to the recycle bin and leave it there until you're sure no problems arise.

Resources: read the optimizing windows articles at www.aumha.org/articles.

Hope this helps.