You might get a small amount of extra space, like really small amount at the cost of somewhat slowing down your browsing, opening, and just about the general windows experience. More resources will need to be used, and some programs cannot run in a compressed state. IT should only be used if you have low amount of hard drive space, and have already performed a disk cleanup disk and deleted unused files and such. I really would not do it
Disk cleanup instructions: Disk Cleanup - Open and Use
A good cleanup and optimization utility:http://www.filehippo.com/download_ccleaner
*warning, using third party registry edits and such can result in corruption and damages. Use at your own risk.* ive tested a lot and used them, and have never came across issues but it has been told, damages could happen.