I got a free Windows 7 disc from Acer as I only purchsed my Laptop from Comet in July. I just went on the Acer Website and found the upgrade link, All I had to do was enter my laptop serial number and pay the delivery costs (£12) and I got my disc a week later.
I think the free upgrades only apply if you purchased windows Vista from around June 2009 onwards, but it also depends on what level of Vista you have. If you have Vista basic, I don't think you can get a free upgrade.
If you are not eligble for a free upgrade, I suspect you will have to pay for it.
By the way, Windows 7 is soooooooo much better than Vista.