Right click on desktop and select personalize. Then Select theme Surface to switch back to default theme.
I had this issue with a client today, the solution was to setup Virtual PC (comes with windows 7). Then install Autocad 2002. After you install the program in the virtual pc, a shortcut direct to the autocad in windows xp mode can be found under the windows 7 system, programs, virtual pc. It was very transparent to my client and he was happy not to have to buy the latest Autocad version.