Thanks sAyer, for pointing me to ADS-Scanner.
The stream is reported having a size of 64 bytes, but content varies. The only thing in common is that they start with "b" as if that should be some kind of signature ?
Here are a few examples:
b…^åDJ
b—YåDÔýVWӷлv¢AVZ5EÜSŸU¥ýêòŠŸVZ5EI•¦,ë³^TI–YåDú\3ÿ®Úüên
b3_åDTh¢ñ_
The ADS can not be removed with the provided button in ADS-Scanner.
Most exe and dll files in my Windows folder are "infected". This is probably due to the big Windows 8.1 November update I downloaded on the 19th. I updated from CIS 7 to CIS 8 on the 18th.
I made a test with a virtual Windows XP machine with CIS 7, following this procedure.
1. Downloaded a exe-file from Internet
2. Ran ADS-Scanner.
No ADS found.
3. Disabled the network.
4. Uninstalled CIS 7.
5. Installed CIS 8.
6. Activated the network.
7. Let CIS 8 update definitions.
8. Downloaded the same exe-file as before
9. Ran ADS-Scanner.
Found an ADS named $CmdZnID, 26 bytes with following content:
[ZoneTransfer]
ZoneId=3
This ADS can be removed.
My Win XP is 32 bit, and my Win 8.1 is 64 bit.
If the ADS on 64 bit is supposed to be something like the one on 32 bit, it may be corrupted ?
This thought made me remember that a few months ago I disabled a setting in Windows 8.1, so zone information should not be saved. Cause I got irritated about Windows warning me about "this program may be unsafe" everytime I had downloaded something.
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Attachments]
SaveZoneInformation=dword:00000001
Maybe that change in registry is causing the corruption ?
I'm thinking of uninstalling CIS 8 and reinstall CIS 7. If that doesn't help I'll restore my C-drive from a backup made on the 17th. But before I do that I'd very much like more input from you, or anyone else.

The biggest issue is that programs I compile myself on my computer also gets the ADS named $CmdTcID.