There are portable virus scanners. Load them into your HDD. ClamWin is one of the best portable antivirus applications. Install it in your external HDD. Simple.
Get clamwin from
here.
And, about password protecting your hard drive, there are no thrid party tools,
AFAIK. But you there are tools from drive vendors which are specific to particular manufacturer or model. I will recommend using TrueCrypt. That will encrypt your volume instead of this password thing.