Welcome to SF. I assume you used two monitors before you upgraded. Please make sure you have the latest video drivers. Problem - I don't see a win 7 driver for this. The x1200 and x1250 has vista drivers.
System Manufacturer/Model Number Dell and Custom OS Systems 1 and 2: Windows 7 Enterprise x64, Win 8 Developer CPU System 1: i7 email@example.comGHz, System 2: AMD FX-4100 Zambezi 3.6G Motherboard System 1:Dell 06NWYK System 2: ASUS M5A97 AM3+ Memory System 1: 8GB System 2: 8GB Graphics Card System 1: ATI FirePro V4800 System 2: Radeon HD 6850 Sound Card System 1: onboard System 2: onboard Monitor(s) Displays System1: Viewsonic HDMI 24" Screen Resolution System 1: 1920x1080 System 2: 1920x1080
Case System 1: Dell System 2: Cooler Master Hard Drives System 1: Mirrored .5B drives System 2: Seagate Barracuda ST1000DM003 1TB 7200 RPM 64MB Cache SATA 6.0Gb/s Internet Speed 10 MBPS