As Greg says, it all depends on hhow you did the upgrade from Vista to Win 7. If you did an 'upgrade install' over Vista, Win 7 will have created a folder called 'windows.old' and all the files from the previous install will be in there.
If you did a 'clean' install, then your hard drive will have been formatted and the file allocation table blanked. This doesn't mean your old files aren't there, or that they can't be recovered. It just means that Windows Explorer cannot see them. If you did do a clean install then you will need recovery software to try and recover your old files.
I have used 'Recuva' from Piriform - www.piriform.com - with some success in the past for this and there are other products to do the same job. However, it is important that you use the computer as little as possible - ideally, not at all - until you are in a position to install and run Recuva, or whatever product you pick. The reason is that your operating system now sees the whole hard drive as a blank slate, and if it decides to store your expenses account in the space where that photo of your favourite cat was, the cat photo is permanently gone.
You are not alone in this - it is said that computer users fall into two groups : those who HAVE lost data; and those who are GOING to lose data. I am firmly in the first group !