Odd - sudoku portable missing, Can not re-install.

I'm not sure what to think - I'm using a toshiba satellite notebook with vista on a secure home network. Had portable sudoku installed for quite some time, when it suddenly disappeared from my computer. The folders and all files except for the .exe file were still there, but twice, the .exe file has disappeared - I was able to re-install the first time, but now, get a consistent error informing me that the folder cannot be opened (I'm installing it in the programs folder on my C: drive). Any idea what may be happening? (Probably not relevant, but I'm using Avast anti-virus app).