I had some difficulties understanding your English but so far I got this

You connected N900 via mass storage mode to copy some content and your friend scanned it with antivirus after that you cant connect N900 via USB at all? Also you have important data on N900 and dont want to reflash it? right?

"One of the USB devices attached to thí computer has manufutioned, and windows does not recogzined it."

This message appears often when you dont safely remove N900 cable when there was an ongoign read/write opertion and now Windows cant recognize it , probably drivers are damaged ....or less likely phone's memory corrupted..anyways u can do two thing

1. Remove all Nokia software from PC , all connectivity drivers, Ovi suite etc...reinstall and try again

2. or you can do a backup on a memory card and format N900, if u dont know how read this wiki

http://wiki.maemo.org/Updating_the_firmware#N900_2

Cheers