which version of Windows 7 you are using ? if 64 bit then there a workaround to successfully install drivers for N900. if using 32-bit then there shouldn't be any problem. and what commands you used to flash your N900 ?

Edit: have you read Power Kernel 52 thread once ? it clearly says that multiboot support in Kernel Power 52 is dropped which means its no longer supported by Kernel Power. that's the reason you are getting Guru Mediation error.