vivmak as travla said the instant disconnect problem could be solved by removing the existing n900 device and then adding it one more time (pairing). the new device will be detected as a phone and if you look at its properties under services you should find that you can enable the keyboard and mouse HID service. If that service was not there then open bluemaemo and try to connect from there this will instigate the service. Then go and select the service and windows will install the drivers for it.

I have to tell you that it was not straight forward for me and i had to experiment with different things but after that it all works perfectly so don't give up.