In my experience this occurs because the ALM Client Registration was previously run on the PC, and now that a patch has been applied to the server, the ALM Client Registration process needs to be run again. This will be an ongoing need for the PC; each time an ALM patch is installed on the server any PC that previously ran the ALM Client Registration will have to do so again. Each patch changes the OTAClient.dll version on the server. If that DLL has been registered on a client system, the ALM Login screen will find it and compare the versions. They must match.
Just using ALM does not register the DLL. This is done when the ALM Client Registration process specifically is run. That process should only need to be run on a system that is attempting to run some sort of integration with ALM, such as integrating other HP tools (i.e. Import from Excel, or QTP) or running a program written to utilize the OTA API.
If such integrations are not being run, and you don't want to have to deal with this problem every time you install an ALM patch, then you should run the process to ''clean'' the PC. Information on this process can be found in HP's knowledge base article http://support.openview.hp.com/selfsolve/document/KM176290