...the files in that area relate to the "profiles" on your PC.
E.g. if jim, geoff and sally all logon to a PC there will be a Jim, Geoff and Sally directory. Plus "All Users" plus "Default User".
Default User for example will have a folder called Start Menu which contains, as I understand it, the default start menu items for your PC - so you don't want to delete that!
Also, your My Documents folder in Windows is stored under there. What you can delete are any profiles that you KNOW you don't use, and also you can empty the contents of the temporary internet files folder by clearing it via Internet Explorer.
Let me know if you need any further info on this.