Slows Down When My Documents Is Accessed

I have the 32bit version and the computer is no more than 2 weeks old. I have a feeling this may be a virus I have always been very careful with what I do on the tinterweb and generally look after the computer as best I can. I have been using Windows Defender firewall and AVG antivirus and spyware.

I downloaded a file last night, placed it inside My Documents> Downloads. I now have reason the believe its a virus. All in all the computer runs like a dream, but since downloading this file, whenever I go into my Documents, my CPU speed jumps up to around 50% and stays there, then If I go into my Downloads file within my documents, my CPU speed shoots up to 100% and hangs there until the computer is restarted? The file that was downloaded is the only file within the Downloads file. I'm 99.9% sure that whatever it is, its causing this problem. If for example I just turn the machine on and dont access my Docs, it runs fine and only slows down when my Documents is accessed. The file that was downloaded is 10mb's, whenever I try to delete it, the dialog just comes up saying calculating time remaining and doesnt budge. I have scanned my docuemnts and no virus was detected by AVG, I just want to get rid of this file but dont know how!