Windows 7 comes with some helpful tools for troubleshooting Aero themes, one of those troubleshooting tools can help you to identify problems with your current display settings that may be prevent you from enabling and running Aero Themes on your PC.
Still, if you find that you are unable to apply the Windows 7 or Windows Vista default theme and that inspite of applying it, you get to see the the Windows Classic theme, here are a few things you may try:
Windows 7 Aero Theme does not work
1) Type Services.msc in Windows start menu search box and hit Enter to open Services. Scroll down to Themes service, and ensure that it is Started and set on Automatic. If not, set it so by right clicking on it and opening the Properties box.
2) Right click on desktop > Personalize > Windows Color and Appearance > Ensure that transparency is checked > OK.
Reboot and try to apply the Theme.
3) Check this post on Aero not working.
If it still doesnt help, I suggest you create a new User Profile, and see if it resolved the issue. If it does solve your problem, you may transfer Personal Data and continue to use this profile.