The trial VM uses Runas.exe to launch IE as a different user. You can use the same approach. When you are login as Administrator, open command prompt and type Runas with the right parameter to launch IE as a different user. Obviously you need to know the password for that account.

See http://technet.microsoft.com/en-us/library/bb490994.aspx for syntax help

K2 process portal is not necessary to start a new instance. You can do the same from the workspace itself. Navigate to Management Console-->Workflow Server-->Process-->Instances-->Start New

Thanks a lot.

I've seen your descriped approach also
in the demo VM. Unfortuneately I didn't manage to test this approach
because I've no LDAP with sample users available. Is there a way to
create sample users. K2 requires a NetBiosName and the LDAP connection
detail. Is it possible to create sample users without the need
of a Sharepoint installation? (Any solution to easily create sample users on the fly?)

The idea of K2 is to use the K2 Process Portal as end user interface. Without Sharepoint only administrators can start new process instances via the Management Console in the workspace. The user tasks of a process can also be handled by the workspace but it's highly recommended to use the Sharepoint integration. Am I right?

K2 is a workflow engine. It does not require SharePoint. K2 works with Active Directory. Open "Active Directory Users and Computers" MMC console and create a new user. I dont understand why you are referring to LDAP and SharePoint to create users.

K2 process portal is NOT an end user interface. It is just the SharePoint version of Mangement Console. Both Process Portal and Management console need K2 Server admin rights.

As I mentioned above, K2 does not require SharePoint to work, so I disagree with your last comment. However, if you already have SharePoint and want to benefit out of it, K2 can integrate with it.

I create exactly the same (start IE via cmd), but I cannot log in K2 workspace (I got only the header and the username from cmd) If I logged via remote connection I can see the workspace. 

Can you tell me what can be the problem?

Thanks
Darko