AVG.
Home user versions of McAfee and Norton are horrid pigs.
They girlfriend's dad's office computers were abysmally slow
The corporate versions (of Mcafee and Norton) are completely different from the home user versions, but you're not going to find them on the shelf at Bust Buy. I'd suggest Symantec Corporate AV, with as many licenses as appropriate. In an office environment you're really supposed to pay for AVG (but it's still really inexpensive).