Well I don't know what to say if you've got Zone Alarm and a backup antivirus scanner. I'd say you have one uniquer problem. Let's run over the problems again and try analzying them once more...
- HDD filled up quickly without sending any files or downloading any files to the HDD
- Antivirus scanners picked up no Antivirus/Spyware
- After Reboot space returned to normal on HDD
- Firewall Zone Alarm is in use
- Computer runs fine at the time being
Am I missing anything?
So from the HDD issue I think that maybe you got a trojan in your HDD somehow, because a "cracker" can implant a virus that fills up all your HDD to distract you and thus by himself time to get what he wants. I think that quite possibly you can someone try to infiltrate your system.
Another thing with that theory is that those types of viruses usually go undetected by Antivirus scanners. Which is bad, so I don't know what to tell you. Try doing a search in the files and folders with the extension .wrm, see what it pulls up. From what I've heard "worms" use the .wrm extension, although wether that's right or not I'm not sure.
Oh well hope everything turns out OK. But what you may want to start doing is deleting all your cookies that contain personal information, get rid of all documents that you feel has anythign personl, basically get rid of everything that has your address, phone number, social security number, your home address, anything that you deem that could get you into harms way, such as identity theft. It doesn't hurt to take action to prptect yourself. If it was a person trying to jack some of your personal information I would STRONGLY
suggest deleting personl information, should the "cracker" try to reeneter your system.
All I could very well be wrong and your PC just had a fluke, but never in the many times trhat I have read PC complaints ahve I encountered anythign such as this. So just take some caution and be on the safe side.