Virus shuts off internet and anti-virus software and causes laptop to crash!!!?

I downloaded a virus I guess. It cuts me off from the internet within minutes of logging on and then it won't even let me online (with ethernet). And when I run AVG the entire system shuts down in the middle of the scan. I can't afford to pay office depot to fix the problem, what should I do? The virus must have been downloaded before I installed AVG because that program is pretty great.