office standard adds PowerPoint to the mix and is valid on a desktop and
portable. I have no idea the price differential. Also, if you own a
qualifying version of a previous office suite, you qualify for upgrade
pricing.
There is a home office & student version which allows 3 installs, without
the single user clause, but it doesn't include outlook. However, it's often
less expensive to buy this and a copy of outlook - especially if you need
word/excel for a spouse or child's computer and they don't use outlook.