You are most likely using QCAD 188.8.131.52 which is from before Windows Vista and known not to work on some Vista installations.
Please update to the latest version from your download area:
-> 2. How can I update to the latest QCAD release?
Please check carefully that you do indeed download QCAD 184.108.40.206. The older versions are also still present in the download area since scripting support is not available in QCAD 220.127.116.11.