Sorry for my English (it's not my native language).
I have a problem with usb-files for windows98se. A flash drive (generic) was not recognized in windows98se, then I replaced all files (.sys) and (.dll) and (.inf) with the "same" from a windowsMe version in their respective system folders. Ooop! Now windows98se don't recognize any usb-driver when I look at the drive manager. What can I do for restore the right drivers and to get my flash drive be functionally active? Thanks a lot for your help.

Try to replace those files with the original one's. If not work maybe try THOSE drivers by Maximus. You can download them HERE.

I recommend [in this *exact* order]:
1. Restore from backup [if any] original 98SE files.
2. If no backup, reinstall 98SE over into the same folder.
3. Install 98SE SP 2.0:
http://www.msfn.org/board/?showtopic=45359
4. Install 98SE2ME:
http://www.msfn.org/board/?showtopic=37302
5. Install Maximus-Decim Native USB 2.0 Drivers:
http://www.msfn.org/board/?showtopic=43605
6. If all of the above doesn't work, install drivers specific for your USB flash stick from their manufacturer web site. Start by installing their drivers made for WinME, if that doesn't work, install their drivers made for Win98SE.

Hope this helps.

PS: This requires that you use English Win98SE, because all these updates/patches are only for English 98SE.