Hi,
Welcome to Seven Forums.
Run Regedit and check the following registry key:
HKEY_LOCAL_MACHINE\Software\Policies\Microsoft\Windows NT\SystemRestore\DisableSR.
If the key is present and has any value other than 0 then system restore is disabled and the system protection tab will be hidden.
Viruses often disable this by setting this registry key. Often they will also disable the task manager and regedit too.
If you get a message saying regedit has been disabled by your administrator then it's quite possible you have been targeted by a virus, in which case a full scan of your system is a good idea.
As always, before making changes to your registry, back it up.
hth
Tanya