can't delete or right-click certain files

This is a strange one: sometimes a file gets created (on the desktop, mostly) that we need to delete. Usually, these files are "saved" emails from Thunderbird - but when we right-click on the file's icon, or we select the file and press the "delete" key, we get an exception (Appname: explorer.ext; Modname: asthisx.dll; exception code: 0xc00000005). We can pick up the file and drag it to the recycle bin and empty the bin, or we can create a folder on the desktop, drag the file to that folder and then right-click/delete the folder (with the offending file in it) with no problem. I've run ccleaner, dial-a-fix, System Mechanic, etc, but the problem remains. We're getting a lot of Thunderbird crashes, and I suspect that it's a Windows problem, as evidenced by the problem with deleting certain files, that's causing the problem in TB. Any ideas?