Might I guess that it's been supplied by your office and the home page is probably that designated by your company?
If so, your system administrator has disabled home page changes and the only way you can change it is to ask him to do it for you (try a beer you never know).
Though, if it is your own personal computer, I can't imagine how you got in this position as it's not a default setting and you're using a Limited User Account. To enable the option again you need to log on using the Administrator Account and remove the Group Policy for this setting from within Administrative Tools.
Look here to learn more.
http://www.microsoft.com/technet/pro....mspx?mfr=true