Need help with a steam issue

I can't seem to load games on steam after downloading them. I've checked the file integrity, restarted my pc, reinstalled the game and steam but no luck so far. I'm trying to play Rocket League and Age of Empires 2 HD. My specs are : Windows 7 ultimate Antec ATX Tower Case PowerUp 550W ATX-e PSU w/ pci-e 6pin + 6pin adapter ASUS P6T7-WS Workstation LGA1366 x58 Motherboard Intel Xeon L5639 6c/12t 2.13GHz @ 2.65GHz Thermaltake Quad heatpipe cooler w/ adjustable fan control DeepCool Z5 Thermal Paste 12GB Kingston HyperX Genesis DDR3-1600 (6*2GB, 2 original kits) 250GB Seagate SATA HD 1TB Seagate SATA HD 2GB HIS IceQ Turbo Radeon HD 6950 SATA DVDRW 120MM Rear Case Exhaust Fan 120MM Side Case Intake fan PCI-E Extreme N Triple Mast Wifi adapter HD Sound Firewire SAS Controller Ethernet

There's no reason these games shouldn't work. I just bought this PC used and it's nice and fast but I can't seem to get games to start. Any help would be appreciated. Thanks.