According to this site (
http://tinyurl.com/26v9sx) it depends on the
version that you bought.
If you have Home or Student edition, it says you can install it on up
to 3 devices. I believe other versions are limited to only 1
installation.
There is a reference on Microsoft site:
http://office.microsoft.com/en-us/products/HA102333141033.aspx?pid=CL100796341033
with one question
Q: I would like to install my copy of Microsoft Office on two desktop
computers. Can I use my portable installation right for the second
install?
A. No, the portable installation right is for use of the primary user
of the first installed copy only and must be on a portable device. A
desktop computer is not a portable device.
So you can install one retail copy of Microsoft on a desktop and a
laptop. If you look at the other questions on this page, the licence
is designed so that one person can use one copy of the software.
Someone else using Office on your laptop at the same time as you use
Office on the desktop machine is breaking the terms of the licence.
If I recall an old licence agreement, you should treat the software
like a book, two people can't read the same book simultaneously.
AFAIK OEM Office version licence is different. One computer only.