How to find if your Mac Book has a Virus?

Hi,

I have a Mac Book pro which is a year old and have been experiencing some strange activity of late. It started by finding random .sgi files showing up in my 'All Documents' file, at first I thought they were for a game I have downloaded, but once I deleted them and it didn't effect my game, I realised that the .sgi files are in fact not from my game. Now I've found my computer lags or freezes a lot and comes up with the rainbow wheel, especially when I am on the internet. I have also found that some how I have over 300 gigs being used on my computer for apps only, however I only have the predownloaded apps and origin and sims. I compared my computer with my sister (who has the same computer with the same apps) and found that she has around 300 gigs used for all genres (Apps, audio, video, etc). The gigabites increase by 1 GB everyday I check the bar. Has anyone else experienced this before? Has anyone found a way to fix it?

Thanks x