How to make to show hidden files

My PC got infected by a virus called I-worm/bagle.

Now, I can not change windows explorer settings to show windows system files or any hidden files – the option which enables to show/hide files in folder options disappeared and (the line in folder file options deleted.

Could you show me how to turn on the option again?
– explorer/tools/folder options/view/files and folders/hidden files and folders/2 choices(do not show/show)

Thanks in advance for our help