bootcamp assistant does not recognise the win7 disk

Hi I'm running Lion 10.7.5 and boot camp 4.0, I'm needing to partitiopn the drive via boot camp assistant, which unfrotunately does not happen automatically according to the info page via apple support.boot camp opens . you download windows support, and thus follows an message

The Startup disk cannot be partitioned or restored to a single partition.(the start up disk must be formatted as a single OS Extended Journaled volume already partitioned by bootcamp assistant for installing windows)

this is as far as the boot camp assistant will allow you to proceed.

Hardware Overview:

Model Name: iMac

Model Identifier: iMac10,1

Processor Name: Intel Core 2 Duo

Processor Speed: 3.33 GHz

Number of Processors: 1

Total Number of Cores: 2

L2 Cache: 6 MB

Memory: 6 GB

Bus Speed: 1.33 GHz

Boot ROM Version: IM101.00CC.B00

SMC Version (system): 1.52f9

Serial Number (system): W894512RB9S

Hardware UUID: 0F008BC0-A309-57CB-B02C-EC6517C1ABAE

I have tried all the processes listed to no avail, if there is something i have missed your input would be appreciated

thanks nutti