unable to create directory

Hi,

I am a real novice with wordpress. I have just installed it. I can do all the things i need to do like put up posts etc. It is the default theme that is in at the moment : Twenty ten

I wanted to change the header. so i created a header of the right size in fireworks and then went back to wordpress>> appearences>> header.

Browsed for the file on my computer and clicked upload. It came back woith this error message:

Unable to create directory D:\inetpub\vhosts\web-writer-articles.co.uk\httpdocs\blog/wp-content/uploads/2010/09. Is its parent directory writable by the server?

Could anyone tell me what the problem may be here?

thanks

Nick