According to Christopher Price of WEEI, the Patriots are scheduled to hold a private workout with Virginia Tech running back Ryan Williams.
Williams isn't the biggest running back, as he stands at just over 5'9" and 212 pounds. Despite this, Williams is actually quite a tough between the tackles runner. He doesn't have elite speed, but he's fast enough. He's got a good burst, and shows enough side to side agility and fluidity in his hips to make defenders miss. He's solid overall and has certain traits that the Patriots will certainly value, he's a good receiver and is more than adequate in pass protection.
One potential downside with Williams could be his health. After an absolutely prolific sophomore campaign that established himself as one of the very best runners in the entire country, Williams struggled with a recurring hamstring injury in 2010 that cost him a good portion of his season, and clearly affected his overall production.
If Williams is healthy, he could very well be the best runner in the entire draft. As of now, he'll need to show the clean bill of health that teams will be looking for. If the Patriots think he can remain healthy, they could select him in the late first/early second round range.