1. Sort by date. The Windows installed items all seem to have a specific date.
2. use http://www.google.com to investigate the rest.
Never delete. Rename so you can use the Recovery Console to put it back.
I'm doing my annual system cleanup, and I know that various programs dump alot of "stuff" into the Windows Directory that may not get deleted with standard uninstall programs and spyware programs.
How can I tell which files are necessary for my current system, and which ones are leftovers from long-gone programs?