HijackThis has got compatibility issues with Windows 7 64-bit. I had the same problems as you and had to uninstall it. As of now, it is open-source software; in other words, TrendMicro seems to have decided to drop HijackThis and stop working on its development.
However: there are other alternatives to HijackThis. Please read tom982's post over here:
What is a good alternative to Trend Micro's HijackThis?
Note: please be very
careful when using such tools. Never
remove/delete anything listed by those tools if you are not familiar with a computer's bowels :-) Instead, save the log (usually a txt file which is automatically generated by the program after the scan) and have it evaluated by someone who knows exactly what they are doing. Deleting the wrong registry entry can result in a dead computer.