Re: XPS 15 fan, on/off/on/off, gahhh!!!
Use Dell Vista drivers for Windows 7, it worked for my NVIDIA, and I had the same fan issues on a Latitude D630 after a rebuild which I solved with updating the BIOS.
If the lastest version Bios is causing the problem maybe try an earlier version.
Latitude D630 T7300 2 Ghz / Windows 7 Pro / 500GB Momentus XT Hybrid HDD Seagate 7200/4GB of solid state SLC NAND flash storage / 4GB Geil 800 Mhz RAM / Custom paint.