If both of your names are down as owners then you own it 50/50, regardless of who contributed what to any downpayment or mortgage payments, as far as I know.
Also Truth, if this is the same woman who ripped you off / ****ed you around last year... then you are a mug.
She could contribute her 25%, become joint owner, and then tell you that she wants to sell up and take her 50% of the value of the property.
Something else to (strongly) consider: If you jointly own a property, she could attempt to take out secured loans (probably for large amounts) against that property. Whilst that technically shouldn't be possible without you agreeing to it and signing the application, some of the "dodgier" companies don't seem to mind bending the rules, and whilst legally you should be in a strong position when it comes to not losing the property, you also have to consider the fact that something like that is likely to go to civil courts and incur court / legal fees.
I'm sorry to not put it in gentler terms but... yeah... you are going to end up being very badly burned if you get yourself financially intertwined with this woman.
If it's NOT her, and is a new woman on the scene then... for goodness sake [n]slow down[/b] man! You can't just go buying houses willy-nilly with women you've only known a matter of weeks
.