I had one on a laptop just the other day. Some symptoms when I checked it out.
1. No exe's / com's ect would run.
2. Nothing in the control panel would run correctly. Device Manager showed an empty window.
3. Ran slow
Mbam, Avast, mbar couldnt find anything wrong. No infections. I fired up my favorite - Rouge Killer and it would not even start. At this point I declared all out war!
I slaved the drive from the laptop to my linux box and checked for hidden partitions. None found. I booted into Kaspersky rescue disk and it didnt find anything.
Finally I got the permissions changed for running exe files (dont ask, dont remember) and ran Rouge Killer. IT found it!! I figure, No wonder it didnt want rouge killer to run eh?
Performed my usual - empty the browser cache/temp directory, Reset DNS, Host file ect ..
I really thought I would find a hidden partition on the laptop. But never found one. I can say though - That zeroday is kinda tricky.
coffee