The Vapi-xp remote agent is downloaded when the ALM client files are installed, if something is wrong with the client files it can cause this error. I would try the following on the machine that you are trying to run the test on:
First run the ALM client uninstaller (if you do not have this program you can get it from your support rep)
Unzip the uninstaller then right click the uninstaller and select ''Run as an administrator''
Select the ''Extended Cleanup'' button and click cleanup
Once this has completed open up a Windows Explorer window and type %appdata% in the address bar and hit enter. This will take you to AppDataRoaming but we need to go to AppDataLocal so click ''AppData'' in the address bar and then click ''Local.'' In the AppDataLocal directory there may still be a folder called ''HP'' open that folder and if you see any folders with a name like ''ALM-Client'' delete those folders. Then in windows explorer type %temp% in the address bar, in this directory look for a folder called ''td_80'' if this folder exists delete it.
Once the above is complete run 32 bit Internet Explorer as an admin (right click run as administrator). If you don't have 32 bit IE you can find it under program files (x86) -> internet explorer -> iexplore.exe.
Once IE is open, set the temporary internet files to ''Check for newer versions of stored pages'' ''Every time I visit the Webpage'' this way the ALM platform loader and associated files are updated when you access the page and the client files will get installed to the correct directory. To set this setting in internet options, under the general tab click settings under Browsing History. Also clear all of your cookies and temporary internet files. Also make sure that the ALM server is added to your trusted sites list in ALM, to do this follow these instructions: http://surfthenetsafely.com/ieseczone7.htm
After this is complete, Navigate to: http://YourQCServer:8080/qcbin then click the Add ins/Tools page link.
Now register the HP ALM Client, click the HP ALM Client Registration link from the add ins/tools page, then click ''Register HP ALM Client'' at the bottom of the page