Many thanks, Essexboy!
My PC seems to be clean now!!!
MBAM and OTS (with the fix script you wrote) have identified the Rootkit Agent, it's name was C:\WINDOWS\system32\drivers\ssvvvlr.sys.
They both tried to delete it immediately and also after a reboot, but they failed.
Then I downloaded Avenger (I found its homepage in 12312's posting in the other thread) and deleted at first only the driver (was Avenger's hint, on its tutorial pages), whose name was ssvvvlr.
Then I could delete already myself, manually the driver file (Rootkit) named above.
Finally, I deleted with Avenger these registry key folders (I couldn't do it manually):
HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Enum\Root\LEGACY_SSVVVLR
HKEY_LOCAL_MACHINE\SYSTEM\ControlSet002\Enum\Root\LEGACY_SSVVVLR
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Enum\Root\LEGACY_SSVVVLR
(though the latter didn't exist anymore when Avenger came to delete it).
Now there are only these two registry keys that contain the hated string:
1.
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Applets\Regedit]
"LastKey"="Arbeitsplatz\\HKEY_LOCAL_MACHINE\\SYSTEM\\ControlSet002\\Enum\\Root\\LEGACY_SSVVVLR"
2.
[HKEY_USERS\S-1-5-21-1482476501-746137067-1957994488-1003\Software\Microsoft\Windows\CurrentVersion\Applets\Regedit]
"LastKey"="Arbeitsplatz\\HKEY_LOCAL_MACHINE\\SYSTEM\\ControlSet002\\Enum\\Root\\LEGACY_SSVVVLR"
But I suppose these keys are harmless and maybe mustn't be deleted, they should get another value soon.
Now MBAM doesn't find any infected file anymore, nor does OTS it.
The internet connection is again fast, there is no sending-receiving of data when I don't do anything, and the sent-received ratio is about 2:7, instead of 1:2.
Thanks again, I wouldn't have solved this without your help!
---
Update:
1. the two remaining registry keys were indeed harmless, they contain now other values.
2. it's worth to start also a full scan with MBAM (not only the quick scan as before), since it detected and deleted me now these items (though, maybe this is only a waste-paper basket?):
C:\System Volume Information\_restore{C91713DB-C2ED-47C6-8A6B-CF59A1ED3B03}\RP2\A0000027.exe (Trojan.Banker) -> Quarantined and deleted successfully.
C:\System Volume Information\_restore{C91713DB-C2ED-47C6-8A6B-CF59A1ED3B03}\RP2\A0000041.exe (Trojan.Banker) -> Quarantined and deleted successfully.
C:\System Volume Information\_restore{C91713DB-C2ED-47C6-8A6B-CF59A1ED3B03}\RP2\A0000043.sys (Rootkit.Agent) -> Quarantined and deleted successfully.
C:\System Volume Information\_restore{C91713DB-C2ED-47C6-8A6B-CF59A1ED3B03}\RP2\A0000052.exe (Trojan.Banker) -> Quarantined and deleted successfully.