Virus and how to fix? Wireless connection HELP please?!

The IT guy at my employer removed some viruses from my computer and told me everything would be OK. This was yesterday. I used my computer for a tad this morning and left it on. I came back and there was a security threat on my screen.

I have a wireless connection. The only website I visted this AM was DG. Is someone getting to my computer thru the wireless connection itself? How do I stop this? When we first got the wireless my daughter set it up, she said there was an option to set up a password but she was never able to find out how to do it.

Any wisdom is appreciated. This is getting very frustrating, it disables my computer and it locks up and it won't even let me shutdown, I have to unplug it.

Thanks (in a pleading tone!)