How can I get win 7 64 bit for old dell laptop?

My current laptop just died so while I shop around for a new one I am forced to use my old Dell Precision M4500 that I bought about 8 years ago. I hadn't used it for a year or two and wasn't working correctly, freezing up right after I started it up, etc. so I decided to do a clean install of windows. When I purchased it years ago it came with windows 7 32 bit installed so the reinstall dvd for windows was for 32 bit as well. After a year or so I upgraded to windows 7 64 bit, the tech department at my university did the upgrade for me.

Now I am in a bit of a jam because when I try to download win 7 64 bit from Microsoft it gives me an error and says because my copy of windows was pre-installed by dell I need to contact them about getting the 64 bit version. But the dell website seems to be telling me that because this computer isn't under warranty I would have to pay to update. 

I can't find any documentation that I upgraded and don't ever remember receiving any (it was 6 or so years ago). Is there any way for me to get my windows 7 64 bit back without paying for it again?

Any help would be appreciated.

Thanks