With the Duet Display app you can use your iPad or iPhone as a secondary display for Windows computers.
“Duet Display allows you to use your iPad or iPhone as an extra display. Developed by a team of ex-Apple engineers, duet is the first high performance solution that has zero lag. Work more effectively with duet. An additional display can increase productivity for engineers, musicians, designers, and artists by up to 48%.”
You can download Duet Display form the App Store: