Hi there,
I am not sure of your exact model, but the one I found based on that description was configured with Windows Vista. Assuming that your recovery partition is still intact, you can use that to reload the factory image. If you are still able to access your hard drive, then try to make a back up of your important data first, if possible. Otherwise it will all be erased in the recovery process.
Once you are ready to go, follow the process to be found here: http://support.hp.com/us-en/product/HP-Pavilion-dv2900-Entertainment-Notebook-PC-series/3709677/model/3759496/document/c00809678/
I post on behalf of HP.