System Restore recovers Registry and System Files and is limited by how much space is allocated to it.
I don't think you will be able to go back beyond the restore points that have fitted into the allocated space.
In Vista it seems to use up the allocated space quickly (and quietly) and slows PC performance. See:http://windows.microsoft.com/en-US/windows...asked-questions
Your slowness may be from old System Restore sets etc:
To clear old System Restore sets - Log in as Administrator, Open COMPUTER, Right click on C: Drive, Select PROPERTIES, Hit DISK CLEANUP and let it do its work. When the results come back Select the MORE OPTIONS tab at the top and then hit CLEAN UP under the System Restore and Shadow Copies heading. Then keep selecting Delete and OK until it has cleaned everything up. You should then Defragment your C: Drive.