If you download from the official site, it's a security/cleaner tool. It's safe, although it could be for advanced users (not simple).
Download ComboFix from
Here or
Here to your Desktop.
Double click combofix.exe and follow the prompts.
When finished, it shall produce a log for you. Post that log and a HiJackthis log in your next reply
Note: Do not mouseclick combofix's window while its running. That may cause it to stall.