File upload problem

Hello,

i started to use Drupal recently and i have a problem with uploading files through Drupal installation interface. I have unchecked 'read only' status of the sites/default/files folder (the destination folders) and all the folders within. It still doesn't work. When i try to upload a file, for example logo image or when i try to add a content to an file field of previously created node it gives me:

• The file ... could not be saved. An unknown error has occured.
• The file in the ... field was unable to be uploaded

Before that i set allowed file extensions in manage fields.
I have Drupal 7 installation on IIS 7

Can somebody give me any suggestion?

thanks