Hi Gerard,
Generally these kind of errors are due to conflict with other running applications on the PC like anti virus.
Try the following steps
Uninstall RTMT
Please Delete the entire Folder for the Registry entries listed below
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\MenuOrder\Start Menu\Programs\Cisco\CallManager Serviceability
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\Real-Time Monitoring Tool x.x (where X are the RTMT Version installed)
HKEY_USERS\S-1-5-21-2543924201-552117928-4032439467-500\Software\Microsoft\Windows\CurrentVersion\Explorer\MenuOrder\StartMenu\Programs\Cisco\CallManagerServiceability
Delete the following folder as well
C:\Program Files\Cisco\CallManager Serviceability\JRtmt\
Once done, restart PC and then attempt a reinstall of RTMT
HTH
Manish