Formatting a laptop... no recovery disk or o/s software

Hi,

This is my own fault I know... but I cannot find any of the installation software for my laptop and need to reformat it.  I actually have three problems:

1. I have a Dell Inspiron 1545 that's out of warranty
2. It came with Vista Home Premium preinstalled
3. I also had a deal to upgrade for free to W7 which I did

So my question is; is it possible to download the system software and also get back to W7, and how do I go about getting it?  I have been going around in circles on Dells website and cannot see anywhere that works... and allows me to download my software.  I did find a place that asked me to register my laptop first, but to do this it wants to install a piece of software on my laptop and as I can't actually use the one I want to fix I don't want to run the software as it will be finding incorrect information.

I have all the details, service tag, model number, service code etc.

Can anyone point me in the right direction?

Thank you

Paul