try this

You can use Microsoft Virtual PC - Also known as XP Mode. Professional, Enterprise, and Ultimate editions of Windows 7 come with Windows XP mode, but must be downloaded and installed before use.
1. Connect your N900 in flasher mode; Hold u on the device keyboard, then connect the USB cable.
2. Wait for Windows to install the appropriate drivers.
3. Start Microsoft Virtual PC / Windows XP mode/
4. Allow the Virtual PC to have access to your N900: (On the top bar): USB > Nokia N900 (Update mode) (Attach). (If it doesn't say (update mode), you didn't enter update mode correctly by pressing "u". Remove the cable and try again.)
5. Wait for Windows to install the appropriate drivers.
6. When it is finished, proceed with the normal flashing steps above.

There have been numerous reports on the forums of people trying to flash the rootfs/FIASCO and eMMC with Windows 7 64 bit XP Mode/Virtual PC, where the rootfs/FIASCO image flashed just fine, but the eMMC image stays at "no suitable device found. Waiting". Again allow the Virtual PC to have access to your N900: (On the top bar): USB > Nokia N900 but *NOTICE* now it will be Nokia N900 (PC-Suite mode) (Attach). Now the flashing will start normally.