I just upgraded to Windows 8, so I went through this myself. I upgraded my Windows 7 install, and linked my Windows account to my Live/MS account. When you link your Windows account to a Live/MS account, the MS account basically takes over. For this purpose, of having a Task or Service run as that user, you use the same local username you normally would use under older Windows versions, but for the password you enter their Live/MS password, not their Windows password.
So let's say you have this setup:
Username - foo
Password - bar
Then you link it to their MS/Live account, and they have this for that account:
Username - [email protected]
Password - xyz
In the Service's Log On tab, for 'This account:' you put in foo, but for password, you put xyz.
If instead you've created a new MS/Live account when you created the user, then the username you use for 'This account:' would be their first name. Their password would be their password. If multiple people share the same first name on your system, go to Control Panel-> Administrative Tools-> Computer Management-> Local Users and Groups-> Users. Each user will have an entry in the list in the center panel. Find the person based on their Full Name in that column. The Name column will provide their Windows Username. Alternatively, their home directory under C:\Users will be their Windows Username.
This also works for Tasks that you've setup to run as a user.