Going on-line without antivirus

Hello,
My free trial period for Norton antivirus has run out and I have no antivirus! How do I go on-line to download a free one or even buy one without running a risk of getting a virus at the same time? the same day it ran out I downloaded avast! and then when I re-started my computer it didnt work normally. I couldn´t even access windows live hotmail. So then I de-installed it and apparently all was normal. The next day however I couldn´t even start my computer as my user profile wouldn´t be accepted. I had to do a system recovery! Could this be a virus? As you can imagine I´m nervous of going on-line and having more problems.
I have a fujitsu siemens laptop with 2G. of RAM working in windows vista.
Thanks for any help.....