You can watch NOWTV in a web browser. Windows 7 doesn't have dedicated apps, for that you need 8, 8.1 or 10.
I pay for a £10 (£1 cheaper if you buy a voucher through GAME rather than direct from Sky) week pass for Sky Sports the weeks that the BBC doesn't have the live races. Luckily my LG TV has a Now TV app built into it, and to be honest, the streaming quality is practically HD. Also use my iPhone/iPad to watch it on the same subscription.