First of all if you have done a clean installation of Windows7, then defrag the hard drive. Otherwise if you have done an upgrade from Vista, then defrag twice. After that verify all your newly uploaded Steam game files. Right click on each game in your Steam Games list, choose Properties> Local Files> verify.
You also have to put your PC in High Performance mode because Windows 7 sets the Performance at Balanced and therefore games will suffer.
Control Panel> Performance and Tools> Adjust Power Settings> Show Additional Plans> High Performance