You can use the registry hack discussed to adjust Aero Peek Desktop Preview Mouse Hover Delay in Windows 7. It's also easily to do.
1 Click the Start menu, and type Regedit.exe in the run box and press ENTER.
2. Navigate to the location as:
HKEY_CURRENT_USER \ Software \ Microsoft \ Windows \ CurrentVersion \ Explorer \ Advanced
3. Create a new DWORD 32bit value and name it as PreviewHoverTime
4. Then double-click PreviewHoverTime and click Decimal toedit the value. I type 3000 in the value data box. 3000 value means that you increase the hover delay to 3 seconds.
5.Close the Registry Editor and logoff.
6.After you login back, you can delay it further more to avoid accidental preview of the desktop in windows 7 now.