When reinstalling windows...it could have installed the wrong video driver. This would cause the problem you are discribing. If you know the right video drivers are installed then make sure they are the latest version. Also since you reinstall all the software that came with your machine you may have some programs auto starting when you boot up your machine. These should be turned off when playing you game. If you have anti-virus software, turn it off before you start you game. If Dell has any kind of system monitoring software running turn that off also.
You could use some more memory. That would make a noticable difference.
