where could I find a .iso file to make a boot-CD which starts antivirus before starting anything else?

http://www.ubcd4win.com/
You'll have to build the iso yourself though.

Avira: http://www.free-av.de/en/tools/12/avira_an...cue_system.html

An other possibility is launching "ClamWin Portable" from a BartPE disk, but you need to be sure the PE is clean.

I MUST recommend BitDefender's Rescue CD as I've not had a SINGLE problem with it. I tried Avira's AntiVir Rescue CD and it wouldn't even detect my Dual SATA RAID 0 Array to scan them!! Kaspersky's Rescue CD only works when you use their AntiVirus product, installed and registered...and F-Secure just isn't reputable/consistent enough in tests to trust...

http://download.bitdefender.com/rescue_cd/..._07_08_2008.iso

My XP SP3 installation uses NOD32, and with BitDefender's occasional boot-time scan, I feel very well protected!!

"Rapture reminds us, we make our own choices, but in the end...our choices make us!!" I trust you'll do what's in your heart...don't just take it from me...

Search for google packs. It includes Norton Antivirus - a standalone. Unlike installable norton products, this is good - it can be used for bartpe or ubcd. it will download the most recent av updates.

also, all the reviews of 2009 norton products have been good as symantec has redesigned all their apps to take on an insignificant footprint. i know this will raise eyebrows here but it's worth a look.