The BIOS of my Dell XPS L702.x was set to AHCI modus. Installing Windows 7 failed.
I fixed this by downloading the latest [Intel Sata Rapid Storage Technology] drivers from the dell support site and extracting the [Intel Sata Rapid Storage Technology v10.1.2.1004_A01.exe] to a USB flash drive.
When installing Microsoft Windows 7, choose Custom installation and load the AHCI driver from the USB flash drive. After that, Microsoft Windows 7 installed just fine.
I am seeing pretty good results on the benchmarks: 556 MB/s Read and 500 MB write: