Explorer.exe Getting "Zeroed Out"!

Okay... this is an odd one.

Something in my system is causing the explorer.exe file to become corrupted and lose all data, hence becoming a "zero byte" file. This, of course, causes the desktop to lose everything but the wallpaper (no shortcut icons, task bar, quick launch...).

The fix is easy enough... I have a working copy of the explorer.exe file on a flash drive and I simply C&P it back to the WINDOWS folder, replacing the zero byte file. This is a bit of a pain and I'm really stumped as to why it's happening in the first place. I've ran several malware programs on the system, but have found nothing.

Anyone have any ideas as to what's going on here?

Thanks!