Try a system restore.
Either tap F8 at startup>safe mode.
Is the start button visable?
Click start>run>type
msconfig
Choose system restore.
Or.
Start > All programs > Accessories > System Tools > System Restore
Select a date just prior to your problem.

Or.
Tap f8 while your system is starting up.
Choose safe mode with command prompt.
Log on as admin.
At command prompt type
%systemroot%\system32\restore\rstrui.exe