What should i do with my Anti-virus program?

Okay, so i was trying to scan my netbook using the McAfee Scanner (Going to the virus scan console and then pressing the full scan button) and then it stops and it says something like this: "The Required DAT files are missing or they are corrupted. Unable to initialize the scanning engine. Reinstall the product" <--- that. The problem is that I CAN'T REINSTALL THE PRODUCT and i really need it. If there's no answer to this DAT problem, can you please find me a FREE INTERNET-DOWNLOADABLE VIRUS EXTINGUISHER PROGRAM? please. Thanks. :)