It's one of the compile tools that processes your map into a .bsp file.
It's ran at the end of the compile, and determines lighting with a process called radiosity. (Hence, RAD)
I've never encountered a problem like this before, so I'm not entirely sure how to fix it.
Are you using Zoner's Compile Tools? By the name qrad, I think you're using the default compile tools that came with Hammer.
Those work, but they don't do their job very well. Zoner's compile tools are optimized, provide good error reports, and have less bugs.
Download them <a href="http://collective.valve-erc.com/index.php?go=mhlt">here</a>, and set up Hammer so it uses them instead.
Hopefully Zoner's tools will fix the problem, or provide an error report telling why this is happening.