Running Windows Powershell Scripts simply opens it in the editor

I'm assuming that Z: is a network drive.

Scheduled jobs are (usually) run under a System account. Network drives aren't usually mapped for System. Even if you change it to run under your credentials (or any specific user credential), you just can't count on network shares being accessible. You'll need to either:

1. Run the second script using a UNC path (and ensure that the system account has access to the UNC path)
2. Map the network drive early on in your script (in PowerShell 3 you could use New-PSDrive, but), the simplest way is to run the NET USE Z: \\SERVER\SHARE command. Of course, you might need to add user credentials: password /USER:domain\user and you could also try adding /SAVECRED /PERSISTENT:YES and running that command once and hoping it sticks (but I'd still test-path and remap it if that's an option).
3. Safest option: make a copy of the file locally.

The problem is that the file you're trying to execute has an alternate file stream that indicates that it came from the Internet. To make the message go away, you merely need to delete the stream. Sysinternals has a free utility called streams.exe that can easily do this for you. (It is important to note, as indicated in the message, that if the file did come from the public Internet, you should be cautious when executing it and make sure you understand completely what it's going to do.)

Once you have streams.exe, the command will be:

streams.exe -d C:\nst\win7.ps1

Viewing the alternate stream from the command line can be tricky, however, notepad++ can open them. On my system, the command would be:

"c:\Program Files (x86)\Notepad++\notepad++.exe" "e:\temp\test.ps1:Zone.Identifier:$DATA"

Also, if you're curious, the stream giving you all the grief contains:

[ZoneTransfer]
ZoneId=3

Defined ZoneIds are:

Local machine = 0
Intranet      = 1
Trusted       = 2
Internet      = 3
Restricted    = 4