1. Boot from CD / USB.
2. Format c:
Or, ano ba balak mo gawen? I suggest to clean the HDD or SDD first, as in wipe using dban or similar. Or just remove all partitions first.
Kung may malapit na computer shop sa inyo, I think na kaya nila ifix yan.
Pero depende pa rin iyan sa may ari ng computer shop kung may services silang ganyan.