If Ewido and the other programs I mentioned before fail to find anything, it may be worth looking for rootkits. There are some free programs available that will remove hidden malware that is otherwise difficult to remove:
F-Secure BlackLight:
http://www.f-secure.com/blacklight
AVG Anti-Rootkit (Beta)
http://www.freewarefiles.com/downloads_counter.php?programid=22524
Sophos Anti-Rootkit:
http://www.sophos.com/products/free-tools/sophos-anti-rootkit.html
UnHackMe is not free but used to have a free working trial, maybe still does:
http://www.greatis.com/unhackme/
If you are unsure about the results of the programs, post them here for advice before proceeding, or open a thread at CastleCops where you will be advised on these and other programs you can run in order to remove rootkits.
http://www.castlecops.com/f233-Rootkit_Revelations.html