Go to MY COMPUTER > (right click) PROPERTIES > SYSTEM RESTORE. Check TURN OFF SYSTEM RESTORE and click APPLY. Run your antivirus to eliminate any remaining viruses. TURN ON SYSTEM RESTORE and adjust the slider to minimum position. Click OK.
I think that the reason you cannot make it minimum is because there are still restore points on your system, if you delete all the restore points, there won't be any restore points using the space and you should be able to move the slider down to minimum.