See the following link , this may help :- http://answers.microsoft.com/en-us/windows/forum/windows_vista-desktop/window-is-maximized-but-not-to-full-screen-size/01a8300c-01b3-4fb2-bbeb-f396ef81c4b3
Let me know if this works.
The answers specified the the above link :-
Answer 1 :-
start closing programs that had notification area icons .
Answer 2 :- application called Pidgin has been identified as the cause in the forum . Closing the application has solved problems for the user.
Alternatively Answer 3 :-
Step 1: Start the System Configuration Utility
Click Start, type msconfig in the Start Search box, and then press ENTER.
If you are prompted for an administrator password or for a confirmation, type the password, or provide confirmation.
Step 2: Configure Selective Startup options
- In the System Configuration Utility dialog box, click Selective Startup on the General tab.
- Click to clear the Load Startup Items check box. Note The Use Original Boot.ini check box is unavailable.
- Click the Services tab.
- Click to select the Hide All Microsoft Services check box.
- Click Disable All, and then click OK.
- When you are prompted, click Restart.
- Create an Alternate Local Administrator account and verify if you are still experiencing this issue.
- Boot into Safe Mode and attempt to maximize your screen and verify if you are still experiencing this issue.