In many cases, re-starting the computer and then re-opening GP4 will correct the error.

If the error still occurs after a re-start, the GP4 program will need to be uninstalled and re-installed.
Windows update must also be run prior to the re-install to make sure all of the system patches and upgrades are installed.

The website for Windows update is: http://windowsupdate.microsoft.com/

The host version of GP4 can be downloaded here: Host Download
The guest version of GP4 can be downloaded here: Guest Download
If after running Windows update and re-installing GP4, you still experience the pure virtual function call error, see this article for instructions on sending your System information file to GatherPlace so we can investigate the issue.