I just looked the file name up on dr web. It says it's a mass mailing worm. I suggest if you have a Antivirus program installed I would run a full scan now. If you don't have one, Avast Home is good. You can run a boot time scan and the program itself scans emails as well as everything else.
I will run dr web on my computer to see if I get it, it may be a false positive.
The themeui.dll is a system file pertaining to Themes.
Thanks for your prompt reply. I did the upgrade from previous build and seems I cought it somewhere on the net. Just restored the original one with "sfc /scannow" command and doing the full hard drive virus scanning...