Not having any antivirus installed on your computer is dangerous as I'm sure you know. With that said, I would recommend you do an online virus scan with one of the Antivirus companies. I've used the Eset Online Scanner many times and it is located here.
Run the online scan and make sure your computer is clean of viruses and the like. Once the scan completes, I'm sure you can find a free antivirus that you can use from this list.
If you continue to have problems after you run the scans, come on back and we'll be glad to help out as best we can.
Hope this helps,