Legally downgrade Windows Vista Home OEM license?

According to official Microsoft document from http://download.microsoft.com/download/5/f/4/5f4c83d3-833e-4f11-8cbd-699b0c164182/royaltyoemreferencesheet.pdf

The OEM vesions of Windows Vista Business and Windows Vista Ultimate include downgrade rights to Microsoft Windows XP Professional, Microsoft Windows XP Professional x64 Edition, and Microsoft Windows XP Tablet PC Edition. Customers may not downgrade to Windows 2000 Professional from Windows Vista Business or Windows Vista Ultimate. No downgrade rights are included in other Windows Vista products in the OEM channel

Translation: Legally you can downgrade OEM versions Vista Business and Vista Ultimate. Other version cannot be legally downgraded.

First an answer I don't believe it's possible for consumers to downgrade a license from Vista Home Premium to XP

There is some info here http://www.computerworld.com/s/article/9040318/FAQ_Giving_up_on_Vista_Here_s_how_to_downgrade_to_XP You'd obviously need to perform a clean install to do this.

Next a comment

Vista isn't crap, it's much better than XP. It had a bad run on release due to hardware manufacturers failing to write half decent drivers.

And some advice

Personally I'd buy a Windows 7 Upgrade license for $50 rather than stepping back 8 years to XP if you really can't stand Vista

Before you even think about going with XP, check whether you can find ALL XP drivers for your PC. Chances are you don't and then the project is dead in the water anyhow. And as was said earlier, who wants to wrestle with an 8 year old OS these days. Follow the advice and get Windows7.