Free, reliable encryption software?

The Internet is a scary place for a Windows user, with every little system being able to infect you... I don't relish the thought of heading into the jungle to find the gem amongst the malware. Can anyone recommend a free, reliable data encryption program I can use to secure my sensitive data on XP Pro? The files will be on an external 250GB HDD, used mainly on my one computer, but every once in a while on a specific other machine.

I would be very grateful if someone could point me in the right direction. Somehow, I don't think archiving WinRAR and setting a password is terribly secure.

 

Thanks for that.
My only problem is, my profile is the one that stays logged in most of the time, and I need to access the files on the drive from another computer, securely. The second one is a sort of a family computer, with no Internet connection, so I'm not worried about attacks. I just want to control access to the files.
I think I need something that can block the files 100% of the time unless you have a password or something similar.
The EFS will definitely help me on my own computer, I think, if I set up a separate profile for accessing the data, but the problem is that I need to get to it occasionally on another machine.
Apologies, this response was written by the Department of Redundancy Department. lol

Do you have any suggestions? Or am I going about this the wrong way?

 

Thanks a lot, I'll take a look at that. From the thread, it seems like this is exactly what I need.