When i start windows, i cannot open any programme nor install a new programme. A box of WINDOWS MEDIA CENTRE is open all the time and all the programme and data and changed to be open with Windows Media Centre.
You should also do a full scan for viruses, an off-line scanner from a CD or flash drive can run on your system even if Windows does not open anything. Check out the Avira Antivir Rescue Disk.
You can also try to do a Repair setup of Windows althogh that is mostly to replace corrupt files, not change settings to default. At the end of all this, if nothing helped, you would need to re-install Windows.