Some web-browsers plugin's do not always work that great in Linux (social stuff most girls like) otherwise its a good platform as already mentioned.
Remember you will be the guy fixing her computer problems regardless
Laptops works sometimes great, sometimes not in Linux ( read driver issues) if you have odd old components in the laptop, desktops are generally better supported.
Best way of finding that out is by testing with a live-CD or live-USB that do not interfere with current installation.
Windows7 is great if you have enough memory - it really like memory, XP is not even supported any more so that excludes it in my book.
Use the 64-but version if you decide for Win7, and if you need - buy more memory.
For windows, buy a good anti-virus that also warns for suspicious websites, and checks downloaded stuff before use.
I like Linux since you can get an old machine running much faster than with Windows, however expect some more internet reading / learning if you get into trouble with Linux than Windows in general.