I had the same issue with 64-bit Windows 7 and VirtualBox some days ago.
This workaround from the wiki installed the right driver:

http://wiki.maemo.org/Updating_the_t...nd_Workarounds

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Issues with 64-bit Windows and Workarounds

According to this post, Windows driver signing can cause problems. You either need to turn on the loading of unsigned drivers or boot your computer to whole another OS. There are a few workarounds:
[edit] Workaround for "no suitable device found" problem on Windows 7 x64

Once this message appears, run inf-wizard.exe from libusb-win32 package and create an inf file for "N900 (PC Suite Mode)" and install this inf driver. Then go to Device Manager and update the driver for N900 Phone Parent and the flasher should spring to life...
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http://sourceforge.net/apps/trac/libusb-win32/wiki