The first Quad Core iX MacBook Pros (2011 - ) use 204-pin 1333 MHz PC3-10600 SO-DIMM DDR3 SDRAM can take up to 16GB via two 8GB modules of 204-pin 1333 MHz PC3-10600 DDR3 SO-DIMM SDRAM. (info)
For future reference in troubleshooting, you don't have to worry about a virus, since there are none. Macs are not immune to malware, but no true viruses exist in the wild that can run on Mac OS X, and there never have been any since it was released 10 years ago. The only malware in the wild that can affect Mac OS X is a handful of trojans, which can be easily avoided with some basic education, common sense and care in what software you install. Also, Mac OS X Snow Leopard and Lion have anti-malware protection built in, further reducing the need for 3rd party antivirus apps.I tried a fresh install thinking it might be a virus too.