Cannot log onto anything that's Yahoo on my computer?

(I'm on my parent's computer) I've tried everything. I have CCleaner and Spyware Terminator to help with spyware and adware. I erase all cookies and history on my computer, yet I still get this page "Cannot access webpage. And whenever I try to find out why, it comes up as "Error Unknown" 0-o Can somebody please give me advice?