I need to replace the belt on a 17.5 Craftsman riding lawn mower and it says to remove the mower? I see the deck the housing and all those other things but, remove the mower? I don't understand.

It would depend on which model you have exactly (number under the seat). But in general, there will be some spring clips on the sides under the footpans. And maybe one hanger with clip (sometimes 2) in front of the mower deck. If the mower has a manual pto clutch, there will be a cable that has to be taken loose as well. If it has an electric pto clutch, you just need to remove the belt.

You didn't specify, but are you trying to replace a deck belt or the drive belt? In most instances, on a craftsman mower that has a model number beginning with 917 you shouldn't need to remove the deck to replace any of the belts.
If you'll post which model you have and which belt you're replacing I can tell you a bit more about it.

The Model is 917.273811 The belt seems to be slipping and doesn't want to cut the grass all the time so I'm going to say the belt is the Ground Belt? Part #130801. It drives fine, just doesn't want to cut all the time.

Thanks for your help

You can adjust the tension on the spring loaded pulley. Inspect, look for a bolt link that can be tightened.

Sears recently set up a new do-it-yourself website: Home Improvement Projects, Product Manuals & Home Management Tools - Managemyhome.com (http://www.managemyhome.com) , it has lots of free manuals for sears products.
On the right side of the home page you'll see "find owners manuals" and a space to type in a model #. I was looking for a manual for an old GT3000 riding mower model #917.275013 and found it there in pdf format... you'll need Adobe Acrobat to download it... the program is also free online.