It looks like your N900 was fine from the start. It seems that it's your pc which is 'infected'. I would advise to stop using your N900 in mass storage mode like that for now. Clean your pc.

It's a pity that now it looks like your N900 is corrupt as well. In your shoes, i'd use Testdisk on N900 or install it in a linux live system, copy all your stuff across, and format MyDocs.