I highly recommend
TeamViewer. There's a free full version available which you need to download on your computer and on the other computer you need to access. Once it's set-up, all you need is the their access code and password for you to be able to sign in and access the other computer remotely. Just take note that the password changes if the pc reboots. I think there's an option for it not to change but I haven't checked on it yet. Anyway, even if the pc is on sleep mode, you can still access it as long as it's on and it's connected to the internet.
I use this on my client's pc whenever I need to check something via Paypal. As I cannot log in to his account on my end.. IP issues and it will deactivate his account for security purposes made by Paypal.
Hope I helped.