Major problems and vital errors after downloading

Question

left the mac downloading from itunes (through firefox) and did not return for a few hours - when i did i found a program called Talkback had installed itself, and the computer seemed to have crashed. i restarted to find a large number of vital errors:
1: finder is non-responsive
2: all documents from hard drive have disappeared/unable to see anything
3:the programmes themselves are working - i can access the internet on firefox for example- (as i am now)
4: when i open Safari this programme TALKBACK appears on the dock (and error messages pop up). it cannot be dragged to the trash
5: all drop down menus are blank (including apple menu), all dialogue boxes are blank
6: all typing apart from in specific apps(ie here!) is blank
7; all desktop files and folders have disappeared
8: fonts have changed style
What can i do? luckily i have everything on an external hard drive, but most of the computer is unusable. What is Talkback and what has it done?
Any help GRATEFULLY recieved.
p.s- i have already posted on this topic but did not give a good title to get attention!

Answer 1

It would really be good if you had the original disks so you could run the Apple hardware test. You can call Apple support and if they still have sets available, they're around $60. You could also buy the TechTool Pro utility which does the same type of hardware testing, as well as has pretty good disk repair functionality, better than what the Apple Disk Utility can handle. When the Apple Disk Utility won't repair a hard drive, Disk Warrior or TechTool Pro are the two choices for repairs.