I have not used Spybot, but I seem to remember reading that it has certain options to impose restrictions on changing home page and accessing internet options from the browser itself.
If that's the cause, decide for yourself whether you want to keep that restriction.
If Spybot is not the cause, you may have to go into the registry. Click Start, Run, and type regedit and press enter. Expand the tree on the left hand side and see if there is a key called:
HKEY Current User\ Software\Policies\ Microsoft\ Internet Explorer\Restrictions \NoBrowserOptions
If you can find it, and the value is 1, then that's the cause. To lift the restriction, either delete the NoBrowserOptions key, or change its value to 0.
Before you make any changes to the registry, back up the key first. Highlight Restrictions on the left hand side, from the top menu click registry, select export registry file, and save it to a location you can remember.