Lots of good places to dine in Loudoun County,Virginia...
Thai - 4912 Thai Cuisine, Sterling (as recommended by others here)
American - Blue Ridge Grill, Leesburg (pricey but tasty)
Pizza - Giovanni's Pizza, Leesburg (authentic tradition NY style)
Chinese - House of Emporor, Ashburn (best Chinese in the county!)
Sushi - Matsuba Sushi, Ashburn (very creative and attentive chef)
Brewhouse - Old Dominion Brew Co, Ashburn (fun place if you can find it)
Italian - Treviso Ristorante, Ashburn (always a good meal)