It will,if left alone,reduce to very sparse use and leave you with a faster hard drive because it is not fragmented. I let mine run and if I use the system it waits till I have finished and then carries on.
if that box is greyed out, i guess you're not allowed, try right click run admin from system tools in start menu. if you're believe in your security software..... 1- Click Start, and type “secpol.msc” in the search area and click Enter. 2- You may receive a prompt from UAC, approve/login and proceed. 3- In the left list, choose “Local Policies”, then “Security Options” 4- Set “Accounts: Administrator account status” to Enabled. 5- Set “User Account Control: Admin Approval Mode for the Built-in Administrator account” to Disabled. 6- Now log-off, and you’ll see a new account named “Administrator” will be available, click on it to login. you will then be allowed:)