Simplify the following text for a Grade 12 native English-speaking person.
Original: In August 2003 , two major worms named the Sobig worm and the Blaster worm began to attack millions of Microsoft Windows computers and caused many problems for system administrators and computer operators who used Windows 2000 .
Simplified: In August 2003 , two major worms called Sobig and Blaster began to attack millions of Microsoft Windows computers , resulting in the largest downtime and clean-up cost to that date .