You will have a hard time with a $400 budget beating a prebuilt from Lenovo, HP, dell, etc.
They get windows for mucch less than the $100 you have to pay.
Spend some time getting rid of the sampleware that they load it with. That also subsidizes their cost.
But, to impress the GF and her mom, I suggest you add your own funds to do something really special.
To my mind, the most important functionalcomponent would be a SSD. It is the one thing that makes everything feel quick.
80gb -120gb might be right. Expect to pay $1 per gb. Perhaps a bit more for a quality unit from Intel or samsung.
A sandy bridge duo will be $50-$100. Really, any will do.
A itx motherboard will be as little as $50.
8gb of ram will be $40
For a case, get a cute one, like the lian li Q07. The red color is outstanding, if you can find one.http://www.newegg.com/Product/Product.aspx?Item=N82E16811112227
Or, a small case like the Antec isk 310-150 which includes a psu.http://www.newegg.com/Product/Product.aspx?Item=N82E16811129081
You get the idea.