HKLM and HKCU are abbreviations often used in virus descriptions to designate Windows Registry Keys HKEY_LOCAL_MACHINE and HKEY_CURRENT_USER. The most common used keys are HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Run and HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run, which specify which programs start automatically when Windows starts.
An .HTA file is an HTML Application web page. It contains scripting code that can create and launch an executable file on the user's disk. It is often used to download Trojans from malicious websites, and is currently being used by Bagle.Q to propagate.
A Boundary Error, also known as a buffer overflow is a software condition when too much data is allowed to be input into a memory area, or value is out of accepted range (like entering 10,000 in a field that only accepts 0 999), and it "crosses the boundary" into other data or code memory. The condition is one of the most common causes of security vulnerabilities.