Download Concurrent_RDP_Win7_7100_32_64.zip to your C:\ and 'Extract All'

Run %windir%\system32\msinfo32.exe to determine your systems build, etc...

Move to c:\Concurrent_RDP_Win7_7100_32_64 then edit the install.cmd file and
adjust 'WINVER', 'PRODUCTNAME' and 'CURRENTBUILD' to match the info from 'System Info', like shown:

SET WINVER=Windows 7 Build 7600
SET PRODUCTNAME="Windows 7 Professional"
SET CURRENTBUILD="7600"
...

Next, in an administrative command prompt move to c:\Concurrent_RDP_Win7_7100_32_64 and execute install.cmd as follows:

install.cmd -blank -multi

When it finnishes if you require blank password logins over the network (Which you really, really shouldn't do!!!)

Run gpedit.msc

Go to 'Computer Configuration > Windows Settings > Security Settings > Local Policies > Security Options'

Set 'Accounts: Limit local accounts use of blank passwords to console login only' to 'Disabled'

Log in from any computer with an RDP client