@Sky:

"Checked" symbols can only be used on a "checked" OS. A "checked" OS version is more fault tolerant than the "free" versions. This is done by stack checking, by using the EBP stack frame to access parameters in all APIs and some other stuff. This makes the OS a bit slower (it is said that it's 5% slower), but helps you with application and driver development a lot since you can easily locate errors. The "checked" Windows builds are only available for MSDN subscribers and only in English. You need special service packs, updates and debug symbols for them, since all code and data is located at different memory locations.

Even if you are a cracker and use a debugger frequently, you still don't need a "checked" build of you OS, since it helps you not or only a bit, depending from what you want to do.