Dell has their Vostro 420 desktop with Windows Vista Business / XP Pro downgrade and 20" LCD monitor for $469. Shipping is $35. For an extra $30 you can upgrade to a 22" LCD monitor and 250GB hard drive.
Additional System Specs:
# 2.8GHz Intel Core 2 Duo Processor
# 160GB SATA Hard Drive
# 2GB DDR2 Memory
# 16x DVD Burner
# 256MB ATI Radeon HD 3450 Video Card